@@ -10,7 +10,7 @@ SYNOPSIS
 --------
 [verse]
 (EXPERIMENTAL!) 'git replay' ([--contained] --onto=<newbase> | --advance=<branch> | --revert=<branch>)
-			     [--ref=<ref>] [--ref-action=<mode>] <revision-range>
+			     [--ref=<ref>] [--ref-action=<mode>] [--linearize] <revision-range>
 
 DESCRIPTION
 -----------

@@ -88,6 +88,25 @@ incompatible with `--contained` (which is a modifier for `--onto` only).
 +
 The default mode can be configured via the `replay.refAction` configuration variable.
 
+--linearize::
+	In this mode, each replayed commit is stacked on top of the
+	previously replayed one, so all replayed commits are flattened into
+	a single linear history.
++
+When a merge commit is encountered, the behavior of git-rebase(1)'s
+option `--no-rebase-merges` is imitated. All commits in the range
+reachable from the merge commit are replayed into a linear history, and
+the merge commit itself is dropped. A ref that pointed to a merge commit
+is updated to the merge's last replayed ancestor.
++
+This flattens the `<revision-range>` as a whole. When multiple revision
+ranges are given they are stacked on top of each other into one linear
+history. Each of their refs is updated to point to its position in that
+history. To linearize ranges separately, replay them in separate `git
+replay` invocations.
++
+This option is incompatible with `--revert`.
+
 <revision-range>::
 	Range of commits to replay; see "Specifying Ranges" in
 	linkgit:git-rev-parse[1]. In `--advance=<branch>` or

@@ -85,7 +85,7 @@ int cmd_replay(int argc,
 	const char *const replay_usage[] = {
 		N_("(EXPERIMENTAL!) git replay "
 		   "([--contained] --onto=<newbase> | --advance=<branch> | --revert=<branch>)\n"
-		   "[--ref=<ref>] [--ref-action=<mode>] <revision-range>"),
+		   "[--ref=<ref>] [--ref-action=<mode>] [--linearize] <revision-range>"),
 		NULL
 	};
 	struct option replay_options[] = {

@@ -111,6 +111,8 @@ int cmd_replay(int argc,
 			     N_("mode"),
 			     N_("control ref update behavior (update|print)"),
 			     PARSE_OPT_NONEG),
+		OPT_BOOL(0, "linearize", &opts.linearize,
+			 N_("drop merge commits, replaying only non-merge commits")),
 		OPT_END()
 	};
 

@@ -132,6 +134,8 @@ int cmd_replay(int argc,
 				  opts.contained, "--contained");
 	die_for_incompatible_opt2(!!opts.ref, "--ref",
 				  !!opts.contained, "--contained");
+	die_for_incompatible_opt2(!!opts.revert, "--revert",
+				  opts.linearize, "--linearize");
 
 	/* Parse ref action mode from command line or config */
 	ref_mode = get_ref_action_mode(repo, ref_action);

@@ -433,26 +433,40 @@ int replay_revisions(struct rev_info *revs,
 	while ((commit = get_revision(revs))) {
 		const struct name_decoration *decoration;
 
-		/*
-		 * Decide where to replay this commit on.
-		 * If the parent commit was replayed already, the replayed result
-		 * can be found in `replayed_commits`. Otherwise fall back to `onto`.
-		 * When reverting, commits are replayed in reverse order and thus
-		 * its parent isn't replayed yet. Therefore revert commits are
-		 * always replayed onto `last_commit`.
-		 */
-		struct commit *parent = commit->parents ? commit->parents->item : NULL;
-		struct commit *base = get_mapped_commit(replayed_commits, parent, onto);
-
-		if (mode == REPLAY_MODE_REVERT)
-			base = last_commit;
-
-		if (commit->parents && commit->parents->next)
-			die(_("replaying merge commits is not supported yet!"));
-
-		last_commit = pick_regular_commit(revs->repo, commit, base,
-						  &merge_opt, &result,
-						  mode, opts->empty);
+		if (commit->parents && commit->parents->next) {
+			if (!opts->linearize)
+				die(_("replaying merge commits is not supported yet!"));
+			/*
+			 * Drop the merge commit: do not pick it, leave
+			 * `last_commit` unchanged, and fall through to the
+			 * rest of the loop. As a result:
+			 * - refs pointing to the merge commit will be updated
+			 *   to `last_commit`.
+			 * - the next replayed commit uses `last_commit` as its
+			 *   `base`.
+			 */
+		} else {
+			/*
+			 * Decide where to replay this commit onto.
+			 * If the parent commit was replayed already, the replayed result
+			 * can be found in `replayed_commits`. Otherwise fall back to `onto`.
+			 * When reverting, commits are replayed in reverse order and thus
+			 * its parent isn't replayed yet. Therefore revert commits are
+			 * always replayed onto `last_commit`.
+			 * Also when opts->linearize is true, set the base to
+			 * `last_commit` to create a single linear history.
+			 */
+			struct commit *parent = commit->parents ? commit->parents->item : NULL;
+			struct commit *base = get_mapped_commit(replayed_commits, parent, onto);
+
+			if (opts->linearize || mode == REPLAY_MODE_REVERT)
+				base = last_commit;
+
+			last_commit = pick_regular_commit(revs->repo, commit, base,
+							  &merge_opt, &result,
+							  mode, opts->empty);
+		}
+
 		if (!last_commit)
 			break;
 

@@ -62,6 +62,11 @@ struct replay_revisions_options {
 	 * Defaults to REPLAY_EMPTY_COMMIT_DROP.
 	 */
 	enum replay_empty_commit_action empty;
+
+	/*
+	 * Whether to linearize the commits (i.e. drop merge commits).
+	 */
+	int linearize;
 };
 
 /* This struct is used as an out-parameter by `replay_revisions()`. */

@@ -52,8 +52,12 @@ test_expect_success 'setup' '
 	test_merge P O --no-ff &&
 	git switch main &&
 
+	git switch --orphan unrelated &&
+	test_commit unrelated-root &&
+
 	git switch -c conflict B &&
-	test_commit C.conflict C.t conflict
+	test_commit C.conflict C.t conflict &&
+	git branch -D unrelated
 '
 
 test_expect_success 'setup bare' '

@@ -97,6 +101,12 @@ test_expect_success '--advance and --contained cannot be used together' '
 	test_grep "cannot be used together" actual
 '
 
+test_expect_success '--revert and --linearize cannot be used together' '
+	test_must_fail git replay --revert=main --linearize \
+		topic1..topic2 2>actual &&
+	test_grep "cannot be used together" actual
+'
+
 test_expect_success 'cannot advance target ... ordering would be ill-defined' '
 	echo "fatal: ${SQ}--advance${SQ} cannot be used with multiple revision ranges because the ordering would be ill-defined" >expect &&
 	test_must_fail git replay --advance=main main topic1 topic2 2>actual &&

@@ -565,4 +575,132 @@ test_expect_success '--onto with --ref rejects multiple revision ranges' '
 	test_grep "cannot be used with multiple revision ranges" err
 '
 
+test_expect_success 'replay to rebase merge commit with --linearize' '
+	git replay --ref-action=print --linearize \
+		--onto main I..topic-with-merge >result &&
+
+	test_line_count = 1 result &&
+
+	git log --format=%s $(cut -f 3 -d " " result) >actual &&
+	test_write_lines O N J M L B A >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'replay to rebase merge commit with --linearize down to the root commit' '
+	git replay --ref-action=print --linearize \
+		--onto unrelated-root topic-with-merge >result &&
+
+	test_line_count = 1 result &&
+
+	git log --format=%s $(cut -f 3 -d " " result) >actual &&
+	test_write_lines O N J I B A unrelated-root >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'replay to cherry-pick merge commit with --linearize' '
+	git replay --ref-action=print --linearize \
+		--advance main I..topic-with-merge >result &&
+
+	test_line_count = 1 result &&
+
+	git log --format=%s $(cut -f 3 -d " " result) >actual &&
+	test_write_lines O N J M L B A >expect &&
+	test_cmp expect actual &&
+
+	printf "update refs/heads/main " >expect &&
+	printf "%s " $(cut -f 3 -d " " result) >>expect &&
+	git rev-parse main >>expect &&
+	test_cmp expect result
+'
+
+test_expect_success 'replay --linearize produces the same patches' '
+	git replay --ref-action=print --linearize \
+		--onto main I..topic-with-merge >result &&
+
+	test_line_count = 1 result &&
+	tip=$(cut -f 3 -d " " result) &&
+
+	# range-diff does not care about the dropped merge,
+	# so the original commits (I..topic-with-merge)
+	# and the replayed chain (main..tip) must produce identical patches.
+	git range-diff I..topic-with-merge main..$tip >out &&
+	test_file_not_empty out &&
+	test_grep ! -v "=" out &&
+
+	git log --oneline main..$tip >out &&
+	test_line_count = 3 out
+'
+
+test_expect_success 'replay with --linearize rebase multiple divergent branches into a single line' '
+	git replay --ref-action=print --linearize \
+		--onto main ^B topic2 topic3 topic4 >result &&
+
+	test_line_count = 3 result &&
+	cut -f 3 -d " " result >new-branch-tips &&
+
+	>expect &&
+	for i in 2 3 4
+	do
+		printf "update refs/heads/topic$i " >>expect &&
+		printf "%s " $(grep topic$i result | cut -f 3 -d " ") >>expect &&
+		git rev-parse topic$i >>expect || return 1
+	done &&
+
+	test_cmp expect result &&
+
+	test_write_lines           E D C M L B A >expect2 &&
+	test_write_lines     H G F E D C M L B A >expect3 &&
+	test_write_lines J I H G F E D C M L B A >expect4 &&
+
+	for i in 2 3 4
+	do
+		git log --format=%s $(grep topic$i result | cut -f 3 -d " ") >actual &&
+		test_cmp expect$i actual || return 1
+	done
+'
+
+test_expect_success 'replay with --linearize of a divergent merge keeps both sides' '
+	test_when_finished "git update-ref -d refs/heads/divergent-x" &&
+	test_when_finished "git update-ref -d refs/heads/divergent-y" &&
+
+	# Build a real merge of two commits that diverged from a common base:
+	#
+	#       X - Z (divergent-x)
+	#      /   /
+	#  M  -  Y (divergent-y)
+	#
+	git switch -c divergent-x main &&
+	test_commit X &&
+	git switch -c divergent-y main &&
+	test_commit Y &&
+	git switch divergent-x &&
+	test_merge Z divergent-y --no-ff &&
+
+	git replay --ref-action=print --linearize \
+		--onto main main..divergent-x >result &&
+	test_line_count = 1 result &&
+	tip=$(cut -f 3 -d " " result) &&
+
+	# The merge Z is dropped, but both X and Y are linearized onto main;
+	# neither side is lost.
+	git log --format=%s main..$tip >actual &&
+	test_write_lines Y X >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '--linearize with --contained updates contained refs' '
+	git replay --ref-action=print --linearize --contained \
+		--onto main ^B topic-with-merge >result &&
+
+	test_line_count = 2 result &&
+
+	git log --format=%s $(head -n 1 result | cut -f 3 -d " ") >actual &&
+	test_write_lines J I M L B A >expect &&
+	test_cmp expect actual &&
+
+	git log --format=%s $(tail -n 1 result | cut -f 3 -d " ") >actual &&
+	test_write_lines O N J I M L B A >expect &&
+	test_cmp expect actual
+'
+
 test_done