clone: Allow repo using gitfile as a reference

Try reading gitfile files when processing --reference options to clone.
This will allow, among other things, using a submodule checked out with
a recent version of git as a reference repository without requiring the
user to have internal knowledge of submodule layout.

Signed-off-by: Aaron Schrab <aaron@schrab.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/clone.c

@@ -232,11 +232,21 @@ static void strip_trailing_slashes(char *dir)
 static int add_one_reference(struct string_list_item *item, void *cb_data)
 {
 	char *ref_git;
+	const char *repo;
 	struct strbuf alternate = STRBUF_INIT;
 
-	/* Beware: real_path() and mkpath() return static buffer */
+	/* Beware: read_gitfile(), real_path() and mkpath() return static buffer */
 	ref_git = xstrdup(real_path(item->string));
-	if (is_directory(mkpath("%s/.git/objects", ref_git))) {
+
+	repo = read_gitfile(ref_git);
+	if (!repo)
+		repo = read_gitfile(mkpath("%s/.git", ref_git));
+	if (repo) {
+		free(ref_git);
+		ref_git = xstrdup(repo);
+	}
+
+	if (!repo && is_directory(mkpath("%s/.git/objects", ref_git))) {
 		char *ref_git_git = mkpathdup("%s/.git", ref_git);
 		free(ref_git);
 		ref_git = ref_git_git;

t/t5700-clone-reference.sh

@@ -185,4 +185,17 @@ test_expect_success 'fetch with incomplete alternates' '
 	! grep " want $tag_object" "$U.K"
 '
 
+test_expect_success 'clone using repo with gitfile as a reference' '
+	git clone --separate-git-dir=L A M &&
+	git clone --reference=M A N &&
+	echo "$base_dir/L/objects" >expected &&
+	test_cmp expected "$base_dir/N/.git/objects/info/alternates"
+'
+
+test_expect_success 'clone using repo pointed at by gitfile as reference' '
+	git clone --reference=M/.git A O &&
+	echo "$base_dir/L/objects" >expected &&
+	test_cmp expected "$base_dir/O/.git/objects/info/alternates"
+'
+
 test_done