perftweaks: unbreak the branchcache code

Summary:
D1451 changed _branchmapupdate() so that it no longer updates self.filteredhash
when changing self.tiprev. This causes branchcache.validfor() to report that
the cache is no longer valid, causing an assertion failure at the end of
updatecache().

Test Plan: Running hg commands no longer crash.

Reviewers: quark, durham, #fbhgext

Reviewed By: quark, #fbhgext

Differential Revision: https://phab.mercurial-scm.org/D1495