verify: use mercurials worker API to speedup stupid verify We use mercurial internal worker api to distribute the workload during verify. This helps with larger repositories. with patch, intel i5 dualcore hyperthreading: $ time hg svn verify hg svn 169.47s user 21.27s system 373% cpu 51.050 total before: $ time hg svn verify hg svn 100.48s user 11.85s system 99% cpu 1:52.97 total This makes stupid verify as fast as subvertpy verify. We use \0 as a separator for serializing the data as it's reserved on common fs.