revset: add optional offset argument to limit() predicate
It's common for GUI or web frontend to fetch chunk of revisions per batch
size. Previously it was possible only if revisions were sorted by revision
number.
$ hg log -r 'limit({revspec} & :{last_known}, 101)'
So this patch introduces a general way to retrieve chunk of revisions after
skipping offset revisions.
$ hg log -r 'limit({revspec}, 100, {last_count})'
This is a dumb implementation. We can optimize it for baseset and spanset
later.