commit-graph: add --changed-paths option to write subcommand

Add --changed-paths option to git commit-graph write. This option will
allow users to compute information about the paths that have changed
between a commit and its first parent, and write it into the commit graph
file. If the option is passed to the write subcommand we set the
COMMIT_GRAPH_WRITE_BLOOM_FILTERS flag and pass it down to the
commit-graph logic.

Helped-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Garima Singh <garima.singh@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit-graph.txt

@@ -57,6 +57,11 @@ or `--stdin-packs`.)
 With the `--append` option, include all commits that are present in the
 existing commit-graph file.
 +
+With the `--changed-paths` option, compute and write information about the
+paths changed between a commit and it's first parent. This operation can
+take a while on large repositories. It provides significant performance gains
+for getting history of a directory or a file with `git log -- <path>`.
++
 With the `--split` option, write the commit-graph as a chain of multiple
 commit-graph files stored in `<dir>/info/commit-graphs`. The new commits
 not already in the commit-graph are added in a new "tip" file. This file

builtin/commit-graph.c

@@ -9,7 +9,7 @@
 
 static char const * const builtin_commit_graph_usage[] = {
 	N_("git commit-graph verify [--object-dir <objdir>] [--shallow] [--[no-]progress]"),
-	N_("git commit-graph write [--object-dir <objdir>] [--append|--split] [--reachable|--stdin-packs|--stdin-commits] [--[no-]progress] <split options>"),
+	N_("git commit-graph write [--object-dir <objdir>] [--append|--split] [--reachable|--stdin-packs|--stdin-commits] [--changed-paths] [--[no-]progress] <split options>"),
 	NULL
 };
 
@@ -19,7 +19,7 @@ static const char * const builtin_commit_graph_verify_usage[] = {
 };
 
 static const char * const builtin_commit_graph_write_usage[] = {
-	N_("git commit-graph write [--object-dir <objdir>] [--append|--split] [--reachable|--stdin-packs|--stdin-commits] [--[no-]progress] <split options>"),
+	N_("git commit-graph write [--object-dir <objdir>] [--append|--split] [--reachable|--stdin-packs|--stdin-commits] [--changed-paths] [--[no-]progress] <split options>"),
 	NULL
 };
 
@@ -32,6 +32,7 @@ static struct opts_commit_graph {
 	int split;
 	int shallow;
 	int progress;
+	int enable_changed_paths;
 } opts;
 
 static struct object_directory *find_odb(struct repository *r,
@@ -135,6 +136,8 @@ static int graph_write(int argc, const char **argv)
 			N_("start walk at commits listed by stdin")),
 		OPT_BOOL(0, "append", &opts.append,
 			N_("include all commits already in the commit-graph file")),
+		OPT_BOOL(0, "changed-paths", &opts.enable_changed_paths,
+			N_("enable computation for changed paths")),
 		OPT_BOOL(0, "progress", &opts.progress, N_("force progress reporting")),
 		OPT_BOOL(0, "split", &opts.split,
 			N_("allow writing an incremental commit-graph file")),
@@ -168,6 +171,8 @@ static int graph_write(int argc, const char **argv)
 		flags |= COMMIT_GRAPH_WRITE_SPLIT;
 	if (opts.progress)
 		flags |= COMMIT_GRAPH_WRITE_PROGRESS;
+	if (opts.enable_changed_paths)
+		flags |= COMMIT_GRAPH_WRITE_BLOOM_FILTERS;
 
 	read_replace_refs = 0;
 	odb = find_odb(the_repository, opts.obj_dir);