tags: cache `repo.changelog` access when checking tags nodes

The tags reading process checks if the nodes referenced in tags exist. Caching
the access to `repo.changelog` provides a large speedup for repositories with
many tags.

running `hg perftags` in a large private repository
before: ! wall 0.393464 comb 0.390000 user 0.330000 sys 0.060000 (median of 25)
after:  ! wall 0.267711 comb 0.270000 user 0.210000 sys 0.060000 (median of 38)

mercurial/localrepo.py

@@ -1417,10 +1417,11 b' class localrepository(object):'
             tags, tt = self._findtags()
         else:
             tags = self._tagscache.tags
+        rev = self.changelog.rev
         for k, v in tags.iteritems():
             try:
                 # ignore tags to unknown nodes
-                self.changelog.rev(v)
+                rev(v)
                 t[k] = v
             except (error.LookupError, ValueError):
                 pass