Merge branch 'pw/sequencer-recover-from-unlockable-index' into maint

The sequencer machinery (used by "git cherry-pick A..B", and "git
rebase -i", among other things) would have lost a commit if stopped
due to an unlockable index file, which has been fixed.

* pw/sequencer-recover-from-unlockable-index:
  sequencer: reschedule pick if index can't be locked

sequencer.c

@@ -438,7 +438,8 @@ static int do_recursive_merge(struct commit *base, struct commit *next,
 	char **xopt;
 	static struct lock_file index_lock;
 
-	hold_locked_index(&index_lock, LOCK_DIE_ON_ERROR);
+	if (hold_locked_index(&index_lock, LOCK_REPORT_ON_ERROR) < 0)
+		return -1;
 
 	read_cache();