merge: make "labels" argument to graft() optional, like it is for update()

graft() just passes the argument on to update(), and update() doesn't
require it, so graft() shouldn't either.

Differential Revision: https://phab.mercurial-scm.org/D6175

mercurial/merge.py

@@ -2205,7 +2205,7 b' def update(repo, node, branchmerge, forc'
                   error=stats.unresolvedcount)
     return stats
 
-def graft(repo, ctx, pctx, labels, keepparent=False,
+def graft(repo, ctx, pctx, labels=None, keepparent=False,
           keepconflictparent=False):
     """Do a graft-like merge.