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