When locking in a symlinked repository, try to lock the original.

In a working tree prepared in new-workdir (in contrib/), some files in .git/
directory are symbolic links to the original repository.  The usual sequence of
lock-write-rename would break the symbolic link.

Ideally we should resolve relative symbolic link with maxdepth, but I do not
want to risk too elaborate patch before 1.5.3 release, so this is a minimum
and trivially obvious fix.  new-workdir creates its symbolic links absolute,
and does not link from a symlinked workdir, so this fix should suffice for now.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

lockfile.c

@@ -28,6 +28,19 @@ static void remove_lock_file_on_signal(int signo)
 static int lock_file(struct lock_file *lk, const char *path)
 {
 	int fd;
+	struct stat st;
+
+	if ((!lstat(path, &st)) && S_ISLNK(st.st_mode)) {
+		ssize_t sz;
+		static char target[PATH_MAX];
+		sz = readlink(path, target, sizeof(target));
+		if (sz < 0)
+			warning("Cannot readlink %s", path);
+		else if (target[0] != '/')
+			warning("Cannot lock target of relative symlink %s", path);
+		else
+			path = target;
+	}
 	sprintf(lk->filename, "%s.lock", path);
 	fd = open(lk->filename, O_RDWR | O_CREAT | O_EXCL, 0666);
 	if (0 <= fd) {