verify: remove dependence on repo.changectx()

This was one of few remaining uses of repo.changectx() in core.

Differential Revision: https://phab.mercurial-scm.org/D3034

mercurial/verify.py

@@ -47,7 +47,7 b' class verifier(object):'
         self.havecl = len(repo.changelog) > 0
         self.havemf = len(repo.manifestlog._revlog) > 0
         self.revlogv1 = repo.changelog.version != revlog.REVLOGV0
-        self.lrugetctx = util.lrucachefunc(repo.changectx)
+        self.lrugetctx = util.lrucachefunc(repo.__getitem__)
         self.refersmf = False
         self.fncachewarned = False
         # developer config: verify.skipflags