cmdutil: rewrite walkchangerevs() by using logcmdutil functions cmdutil.walkchangerevs() now takes (revs, makefilematcher) in place of (match, opts), and only provides the "windowing" functionality. Unused classes and functions will be removed by the next patch. "hg grep --follow" (--all-files) is still broken since there is no logic to follow copies while traversing changelog, but at least, it does follow the DAG.