Handle relative paths in submodule .git files

Commit 842abf0 (Teach resolve_gitlink_ref() about the .git file, 2008-02-20)
taught resolve_gitlink_ref() to call read_gitfile_gently() to resolve .git
files.  In this commit teach read_gitfile_gently() to interpret a relative
path in a .git file with respect to the file location.

This change allows update-index to recognize a submodule that uses a relative
path in its .git file.  It previously failed because the relative path was
wrongly interpreted with respect to the superproject directory.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -252,6 +252,8 @@ static int check_repository_format_gently(int *nongit_ok)
 const char *read_gitfile_gently(const char *path)
 {
 	char *buf;
+	char *dir;
+	const char *slash;
 	struct stat st;
 	int fd;
 	size_t len;
@@ -276,9 +278,23 @@ const char *read_gitfile_gently(const char *path)
 	if (len < 9)
 		die("No path in gitfile: %s", path);
 	buf[len] = '\0';
-	if (!is_git_directory(buf + 8))
-		die("Not a git repository: %s", buf + 8);
-	path = make_absolute_path(buf + 8);
+	dir = buf + 8;
+
+	if (!is_absolute_path(dir) && (slash = strrchr(path, '/'))) {
+		size_t pathlen = slash+1 - path;
+		size_t dirlen = pathlen + len - 8;
+		dir = xmalloc(dirlen + 1);
+		strncpy(dir, path, pathlen);
+		strncpy(dir + pathlen, buf + 8, len - 8);
+		dir[dirlen] = '\0';
+		free(buf);
+		buf = dir;
+	}
+
+	if (!is_git_directory(dir))
+		die("Not a git repository: %s", dir);
+	path = make_absolute_path(dir);
+
 	free(buf);
 	return path;
 }

t/t2104-update-index-gitfile.sh

@@ -31,7 +31,7 @@ test_expect_success 'submodule with relative .git file' '
 	 test_commit first)
 '
 
-test_expect_failure 'add gitlink to relative .git file' '
+test_expect_success 'add gitlink to relative .git file' '
 	git update-index --add -- sub2
 '