set_worktree_head_symref(): fix error message

Emit an informative error when failed to hold lock of HEAD.

2233066e (refs: add a new function set_worktree_head_symref,
2016-03-27) added set_worktree_head_symref(), but this is missing a
call to unable_to_lock_message() after hold_lock_file_for_update()
fails, so it emits an empty error message:

  % git branch -m oldname newname
  error:
  error: HEAD of working tree /path/to/wt is not updated
  fatal: Branch renamed to newname, but HEAD is not updated!

Thanks to Eric Sunshine for pointing this out.

Signed-off-by: Kazuki Yamaguchi <k@rhe.jp>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/files-backend.c

@@ -2898,7 +2898,6 @@ int set_worktree_head_symref(const char *gitdir, const char *target)
 {
 	static struct lock_file head_lock;
 	struct ref_lock *lock;
-	struct strbuf err = STRBUF_INIT;
 	struct strbuf head_path = STRBUF_INIT;
 	const char *head_rel;
 	int ret;
@@ -2906,6 +2905,8 @@ int set_worktree_head_symref(const char *gitdir, const char *target)
 	strbuf_addf(&head_path, "%s/HEAD", absolute_path(gitdir));
 	if (hold_lock_file_for_update(&head_lock, head_path.buf,
 				      LOCK_NO_DEREF) < 0) {
+		struct strbuf err = STRBUF_INIT;
+		unable_to_lock_message(head_path.buf, errno, &err);
 		error("%s", err.buf);
 		strbuf_release(&err);
 		strbuf_release(&head_path);