branchcache: don't verify closed nodes in iteropen()
We expect that the nodes passed to iteropen() will be verified. We are only
testing for membership in closed nodes set, so we don't need to verify the whole
closed nodes set.
This will speed up calculating branchheads() when there are lot of closed nodes
related to other branches.
Differential Revision: https://phab.mercurial-scm.org/D6209
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