pack-objects: disable --full-name-hash when shallow

As demonstrated in the previous change, the --full-name-hash option of
'git pack-objects' is less effective in a trunctated history. Thus, even
when the option is selected via a command-line option or config, disable
this option when the '--shallow' option is specified. This will help
performance in servers that choose to enable the --full-name-hash option
by default for a repository while not regressing their ability to serve
shallow clones.

This will not present a compatibility issue in the future when the full
name hash values are stored in the reachability bitmaps, since shallow
clones disable bitmaps.

Signed-off-by: Derrick Stolee <stolee@gmail.com>

builtin/pack-objects.c

@@ -4589,6 +4589,12 @@ int cmd_pack_objects(int argc,
 	if (use_full_name_hash < 0)
 		use_full_name_hash = git_env_bool("GIT_TEST_FULL_NAME_HASH", 0);
 
+	if (shallow && use_full_name_hash > 0 &&
+	    !git_env_bool("GIT_TEST_USE_FULL_NAME_HASH_WITH_SHALLOW", 0)) {
+		use_full_name_hash = 0;
+		warning("the --full-name-hash option is disabled with the --shallow option");
+	}
+
 	if (write_bitmap_index && use_full_name_hash > 0) {
 		warning(_("currently, the --full-name-hash option is incompatible with --write-bitmap-index"));
 		use_full_name_hash = 0;

t/perf/p5313-pack-objects.sh

@@ -66,6 +66,7 @@ test_size 'shallow pack size' '
 '
 
 test_perf 'shallow pack with --full-name-hash' '
+	GIT_TEST_USE_FULL_NAME_HASH_WITH_SHALLOW=1 \
 	git pack-objects --stdout --revs --sparse --shallow --full-name-hash <in-shallow >out
 '