gc: add gc.repackFilter config option

A previous commit has implemented `git repack --filter=<filter-spec>` to
allow users to filter out some objects from the main pack and move them
into a new different pack.

Users might want to perform such a cleanup regularly at the same time as
they perform other repacks and cleanups, so as part of `git gc`.

Let's allow them to configure a <filter-spec> for that purpose using a
new gc.repackFilter config option.

Now when `git gc` will perform a repack with a <filter-spec> configured
through this option and not empty, the repack process will be passed a
corresponding `--filter=<filter-spec>` argument.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/gc.txt

@@ -145,6 +145,11 @@ Multiple hooks are supported, but all must exit successfully, else the
 operation (either generating a cruft pack or unpacking unreachable
 objects) will be halted.
 
+gc.repackFilter::
+	When repacking, use the specified filter to move certain
+	objects into a separate packfile.  See the
+	`--filter=<filter-spec>` option of linkgit:git-repack[1].
+
 gc.rerereResolved::
 	Records of conflicted merge you resolved earlier are
 	kept for this many days when 'git rerere gc' is run.

builtin/gc.c

@@ -61,6 +61,7 @@ static timestamp_t gc_log_expire_time;
 static const char *gc_log_expire = "1.day.ago";
 static const char *prune_expire = "2.weeks.ago";
 static const char *prune_worktrees_expire = "3.months.ago";
+static char *repack_filter;
 static unsigned long big_pack_threshold;
 static unsigned long max_delta_cache_size = DEFAULT_DELTA_CACHE_SIZE;
 
@@ -170,6 +171,8 @@ static void gc_config(void)
 	git_config_get_ulong("gc.bigpackthreshold", &big_pack_threshold);
 	git_config_get_ulong("pack.deltacachesize", &max_delta_cache_size);
 
+	git_config_get_string("gc.repackfilter", &repack_filter);
+
 	git_config(git_default_config, NULL);
 }
 
@@ -355,6 +358,9 @@ static void add_repack_all_option(struct string_list *keep_pack)
 
 	if (keep_pack)
 		for_each_string_list(keep_pack, keep_one_pack, NULL);
+
+	if (repack_filter && *repack_filter)
+		strvec_pushf(&repack, "--filter=%s", repack_filter);
 }
 
 static void add_repack_incremental_option(void)

t/t6500-gc.sh

@@ -202,6 +202,19 @@ test_expect_success 'one of gc.reflogExpire{Unreachable,}=never does not skip "e
 	grep -E "^trace: (built-in|exec|run_command): git reflog expire --" trace.out
 '
 
+test_expect_success 'gc.repackFilter launches repack with a filter' '
+	test_when_finished "rm -rf bare.git" &&
+	git clone --no-local --bare . bare.git &&
+
+	git -C bare.git -c gc.cruftPacks=false gc &&
+	test_stdout_line_count = 1 ls bare.git/objects/pack/*.pack &&
+
+	GIT_TRACE=$(pwd)/trace.out git -C bare.git -c gc.repackFilter=blob:none \
+		-c repack.writeBitmaps=false -c gc.cruftPacks=false gc &&
+	test_stdout_line_count = 2 ls bare.git/objects/pack/*.pack &&
+	grep -E "^trace: (built-in|exec|run_command): git repack .* --filter=blob:none ?.*" trace.out
+'
+
 prepare_cruft_history () {
 	test_commit base &&