@@ -41,6 +41,9 @@ With the `--stdin-commits` option, generate the new commit graph by
 walking commits starting at the commits specified in stdin as a list
 of OIDs in hex, one OID per line. (Cannot be combined with
 --stdin-packs.)
++
+With the `--additive` option, include all commits that are present
+in the existing commit-graph file.
 
 'read'::
 

@@ -70,6 +73,13 @@ $ echo <pack-index> | git commit-graph write --stdin-packs
 $ git show-ref -s | git commit-graph write --stdin-commits
 ------------------------------------------------
 
+* Write a graph file containing all commits in the current
+* commit-graph file along with those reachable from HEAD.
++
+------------------------------------------------
+$ git rev-parse HEAD | git commit-graph write --stdin-commits --additive
+------------------------------------------------
+
 * Read basic information from the commit-graph file.
 +
 ------------------------------------------------

@@ -8,7 +8,7 @@
 static char const * const builtin_commit_graph_usage[] = {
 	N_("git commit-graph [--object-dir <objdir>]"),
 	N_("git commit-graph read [--object-dir <objdir>]"),
-	N_("git commit-graph write [--object-dir <objdir>] [--stdin-packs|--stdin-commits]"),
+	N_("git commit-graph write [--object-dir <objdir>] [--additive] [--stdin-packs|--stdin-commits]"),
 	NULL
 };
 

@@ -18,7 +18,7 @@ static const char * const builtin_commit_graph_read_usage[] = {
 };
 
 static const char * const builtin_commit_graph_write_usage[] = {
-	N_("git commit-graph write [--object-dir <objdir>] [--stdin-packs|--stdin-commits]"),
+	N_("git commit-graph write [--object-dir <objdir>] [--additive] [--stdin-packs|--stdin-commits]"),
 	NULL
 };
 

@@ -26,6 +26,7 @@ static struct opts_commit_graph {
 	const char *obj_dir;
 	int stdin_packs;
 	int stdin_commits;
+	int additive;
 } opts;
 
 static int graph_read(int argc, const char **argv)

@@ -94,6 +95,8 @@ static int graph_write(int argc, const char **argv)
 			N_("scan packfiles listed by stdin for commits")),
 		OPT_BOOL(0, "stdin-commits", &opts.stdin_commits,
 			N_("start walk at commits listed by stdin")),
+		OPT_BOOL(0, "additive", &opts.additive,
+			N_("include all commits already in the commit-graph file")),
 		OPT_END(),
 	};
 

@@ -131,7 +134,8 @@ static int graph_write(int argc, const char **argv)
 			   pack_indexes,
 			   packs_nr,
 			   commit_hex,
-			   commits_nr);
+			   commits_nr,
+			   opts.additive);
 
 	return 0;
 }

@@ -533,7 +533,8 @@ void write_commit_graph(const char *obj_dir,
 			const char **pack_indexes,
 			int nr_packs,
 			const char **commit_hex,
-			int nr_commits)
+			int nr_commits,
+			int additive)
 {
 	struct packed_oid_list oids;
 	struct packed_commit_list commits;

@@ -551,10 +552,24 @@ void write_commit_graph(const char *obj_dir,
 	oids.nr = 0;
 	oids.alloc = approximate_object_count() / 4;
 
+	if (additive) {
+		prepare_commit_graph_one(obj_dir);
+		if (commit_graph)
+			oids.alloc += commit_graph->num_commits;
+	}
+
 	if (oids.alloc < 1024)
 		oids.alloc = 1024;
 	ALLOC_ARRAY(oids.list, oids.alloc);
 
+	if (additive && commit_graph) {
+		for (i = 0; i < commit_graph->num_commits; i++) {
+			const unsigned char *hash = commit_graph->chunk_oid_lookup +
+				commit_graph->hash_len * i;
+			hashcpy(oids.list[oids.nr++].hash, hash);
+		}
+	}
+
 	if (pack_indexes) {
 		struct strbuf packname = STRBUF_INIT;
 		int dirlen;

@@ -40,7 +40,8 @@ void write_commit_graph(const char *obj_dir,
 			const char **pack_indexes,
 			int nr_packs,
 			const char **commit_hex,
-			int nr_commits);
+			int nr_commits,
+			int additive);
 
 #endif
 

@@ -190,6 +190,16 @@ test_expect_success 'build graph from commits with closure' '
 graph_git_behavior 'graph from commits, commit 8 vs merge 1' full commits/8 merge/1
 graph_git_behavior 'graph from commits, commit 8 vs merge 2' full commits/8 merge/2
 
+test_expect_success 'build graph from commits additively' '
+        cd "$TRASH_DIRECTORY/full" &&
+	git rev-parse merge/3 | git commit-graph write --stdin-commits --additive &&
+	test_path_is_file $objdir/info/commit-graph &&
+	graph_read_expect "10" "large_edges"
+'
+
+graph_git_behavior 'additive graph, commit 8 vs merge 1' full commits/8 merge/1
+graph_git_behavior 'additive graph, commit 8 vs merge 2' full commits/8 merge/2
+
 test_expect_success 'setup bare repo' '
         cd "$TRASH_DIRECTORY" &&
 	git clone --bare --no-local full bare &&