Martin von Zweigbergk <martinvonz@google.com> [Fri, 20 Jul 2018 11:37:31 -0700] rev 38819
perfmoonwalk: make work with filtered repo
This also fixes the out-of-range "len(repo)" that weirdly works (and
gets the nullid). I suspect it wasn't intentional to include the
timing of reading the null revision.
Differential Revision: https://phab.mercurial-scm.org/D4017