rewriteutil: allow rewriting merge commits (issue4561) This patch simply allows rewriteutil.rewrite() to work with commits with multiple parents (i.e. merges). That function is used in such commands as fold, metaedit, touch, rewind. The issue 4561 is marked as easy, the limitation is called unnecessary, no tests fail after this change. What can go wrong.

extrakey = 'topic'
changekey = '_rewrite_noise'