patch-id: use RUN_SETUP_GENTLY

Patch-id does not require a repository because it is just
processing the incoming diff on stdin, but it may look at
git config for keys like patchid.stable.

Even though we do not setup_git_directory(), this works from
the top-level of a repository because we blindly look at
".git/config" in this case. But as the included test
demonstrates, it does not work from a subdirectory.

We can fix it by using RUN_SETUP_GENTLY. We do not take any
filenames from the user on the command line, so there's no
need to adjust them via prefix_filename().
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git.c

@@ -444,7 +444,7 @@ static struct cmd_struct commands[] = {
 	{ "pack-objects", cmd_pack_objects, RUN_SETUP },
 	{ "pack-redundant", cmd_pack_redundant, RUN_SETUP },
 	{ "pack-refs", cmd_pack_refs, RUN_SETUP },
-	{ "patch-id", cmd_patch_id },
+	{ "patch-id", cmd_patch_id, RUN_SETUP_GENTLY },
 	{ "pickaxe", cmd_blame, RUN_SETUP },
 	{ "prune", cmd_prune, RUN_SETUP },
 	{ "prune-packed", cmd_prune_packed, RUN_SETUP },

t/t4204-patch-id.sh

@@ -143,6 +143,20 @@ test_expect_success 'patch-id supports git-format-patch MIME output' '
 	test_cmp patch-id_master patch-id_same
 '
 
+test_expect_success 'patch-id respects config from subdir' '
+	test_config patchid.stable true &&
+	mkdir subdir &&
+
+	# copy these because test_patch_id() looks for them in
+	# the current directory
+	cp bar-then-foo foo-then-bar subdir &&
+
+	(
+		cd subdir &&
+		test_patch_id irrelevant patchid.stable=true
+	)
+'
+
 cat >nonl <<\EOF
 diff --git i/a w/a
 index e69de29..2e65efe 100644