copies: refactor symmetricdifference as _findlimit
We only need to track the lowest revision seen, which makes things simpler.
% Without -v
access log created - .hg/hgrc respected
% With -v
listening at http://localhost/ (127.0.0.1)
% With --prefix foo
listening at http://localhost/foo/ (127.0.0.1)
% With --prefix /foo
listening at http://localhost/foo/ (127.0.0.1)
% With --prefix foo/
listening at http://localhost/foo/ (127.0.0.1)
% With --prefix /foo/
listening at http://localhost/foo/ (127.0.0.1)