Before creating NOTES_MERGE_REF, check NOTES_MERGE_REF using
find_shared_symref and die if we find one. This prevents simultaneous
merges to the same notes branch from different worktrees.
Signed-off-by: David Turner <dturner@twopensource.com>
Reviewed-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
