log: remove redundant handling of --limit A returned 'revs' set is pre-filtered by cmdutil.getlogrevs(). The result of displayer.flush() isn't used anymore, so removed.

log: use smartset.slice() to limit number of revisions to be displayed

log: merge getlogrevs() and getgraphlogrevs() cmdutil.graphlog() is updated to receive (revs, filematcher) as arguments to make sure that opts['graph'] is set when getlogrevs() is invoked.