@@ -102,3 +102,10 @@ for details).
 	`git branch --edit-description`. Branch description is
 	automatically added to the `format-patch` cover letter or
 	`request-pull` summary.
+
+`branch.<name>.pruneMerged`::
+	If set to `false`, branch _<name>_ is exempt from
+	`git branch --prune-merged`.  Useful for a topic branch you
+	intend to develop further after an initial round has been
+	merged upstream.  Defaults to true.  Explicit deletion via
+	`git branch -d` is unaffected.

@@ -226,9 +226,10 @@ the upstream refs refreshed.
 +
 A branch is left alone if any of the following holds:
 its upstream no longer resolves locally; it is checked out in any
-worktree; or its push destination (`<branch>@{push}`) equals its
+worktree; its push destination (`<branch>@{push}`) equals its
 upstream (`<branch>@{upstream}`), so it cannot be distinguished
-from a freshly pulled trunk that just looks "fully merged".
+from a freshly pulled trunk that just looks "fully merged"; or
+`branch.<name>.pruneMerged` is set to `false`.
 +
 Branches refused by the "fully merged" safety check are listed as
 warnings and skipped; pass them to `git branch -D` explicitly if

@@ -875,7 +875,9 @@ static int prune_merged_branches(int argc, const char **argv, int quiet)
 		struct branch *branch = branch_get(short_name);
 		const char *upstream, *push;
 		struct strbuf full = STRBUF_INIT;
+		struct strbuf key = STRBUF_INIT;
 		int skip;
+		int opt_out;
 
 		strbuf_addf(&full, "refs/heads/%s", short_name);
 		skip = !!branch_checked_out(full.buf);

@@ -890,6 +892,18 @@ static int prune_merged_branches(int argc, const char **argv, int quiet)
 		if (!push || !strcmp(push, upstream))
 			continue;
 
+		strbuf_addf(&key, "branch.%s.prunemerged", short_name);
+		if (!repo_config_get_bool(the_repository, key.buf, &opt_out) &&
+		    !opt_out) {
+			if (!quiet)
+				fprintf(stderr,
+					_("Skipping '%s' (branch.%s.pruneMerged is false)\n"),
+					short_name, short_name);
+			strbuf_release(&key);
+			continue;
+		}
+		strbuf_release(&key);
+
 		strvec_push(&deletable, short_name);
 	}
 

@@ -1990,4 +1990,34 @@ test_expect_success '--prune-merged requires at least one <branch>' '
 	test_grep "at least one <branch>" err
 '
 
+test_expect_success '--prune-merged honours branch.<name>.pruneMerged=false' '
+	test_when_finished "rm -rf pm-optout" &&
+	git clone pm-upstream pm-optout &&
+	git -C pm-optout remote add fork ../pm-fork &&
+	test_config -C pm-optout remote.pushDefault fork &&
+	test_config -C pm-optout push.default current &&
+	git -C pm-optout branch one one-commit &&
+	git -C pm-optout branch --set-upstream-to=origin/next one &&
+	git -C pm-optout branch two two-commit &&
+	git -C pm-optout branch --set-upstream-to=origin/next two &&
+	test_config -C pm-optout branch.one.pruneMerged false &&
+
+	git -C pm-optout branch --prune-merged "origin/*" 2>err &&
+
+	git -C pm-optout rev-parse --verify refs/heads/one &&
+	test_must_fail git -C pm-optout rev-parse --verify refs/heads/two &&
+	test_grep "Skipping .one." err
+'
+
+test_expect_success 'branch -d still deletes a pruneMerged=false branch' '
+	test_when_finished "rm -rf pm-optout-d" &&
+	git clone pm-upstream pm-optout-d &&
+	git -C pm-optout-d branch one one-commit &&
+	git -C pm-optout-d branch --set-upstream-to=origin/next one &&
+	test_config -C pm-optout-d branch.one.pruneMerged false &&
+
+	git -C pm-optout-d branch -d one &&
+	test_must_fail git -C pm-optout-d rev-parse --verify refs/heads/one
+'
+
 test_done