enter_repo: avoid duplicating logic, use is_git_directory() instead

It matters for linked checkouts where 'refs' directory won't be
available in $GIT_DIR. is_git_directory() knows about $GIT_COMMON_DIR
and can handle this case.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -441,8 +441,7 @@ const char *enter_repo(const char *path, int strict)
 	else if (chdir(path))
 		return NULL;
 
-	if (access("objects", X_OK) == 0 && access("refs", X_OK) == 0 &&
-	    validate_headref("HEAD") == 0) {
+	if (is_git_directory(".")) {
 		set_git_dir(".");
 		check_repository_format();
 		return path;

t/t0002-gitfile.sh

@@ -134,4 +134,18 @@ test_expect_success 'enter_repo non-strict mode' '
 	test_cmp expected actual
 '
 
+test_expect_success 'enter_repo linked checkout' '
+	(
+		cd enter_repo &&
+		git worktree add  ../foo refs/tags/foo
+	) &&
+	git ls-remote foo >actual &&
+	cat >expected <<-\EOF &&
+	946e985ab20de757ca5b872b16d64e92ff3803a9	HEAD
+	946e985ab20de757ca5b872b16d64e92ff3803a9	refs/heads/master
+	946e985ab20de757ca5b872b16d64e92ff3803a9	refs/tags/foo
+	EOF
+	test_cmp expected actual
+'
+
 test_done