branchcache: lazily validate nodes in iteritems()
This saves ~0.30 sec on creating a new branch on our internal repo.
Differential Revision: https://phab.mercurial-scm.org/D6236
branchcache: only iterate over branches which needs to be verified
Otherwise we loop over all the branches and call _verifybranch() even if not
required.
Differential Revision: https://phab.mercurial-scm.org/D6240