localrepo: forcibly copy list of filecache keys

On Python 3, keys() is more like iterkeys(), so we got in trouble for
mutating the dict while we're iterating here. Since the list of caches
should be relatively small, work around this difference by just
forcing a copy of the key list.

mercurial/localrepo.py

@@ -1318,7 +1318,7 b' class localrepository(object):'
         redundant one doesn't).
         '''
         unfiltered = self.unfiltered() # all file caches are stored unfiltered
-        for k in self._filecache.keys():
+        for k in list(self._filecache.keys()):
             # dirstate is invalidated separately in invalidatedirstate()
             if k == 'dirstate':
                 continue