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>