pushkey: flush pending data before running a pre-pushkey hook (issue4607)

The pre-pushkey hook will likely validate the pushkey based on element
previously changed in the same transaction. We need to make theses data
available for the hook.

mercurial/localrepo.py

@@ -1905,6 +1905,8 b' class localrepository(object):'
             hookargs = {}
             if tr is not None:
                 hookargs.update(tr.hookargs)
+                pending = lambda: tr.writepending() and self.root or ""
+                hookargs['pending'] = pending
             hookargs['namespace'] = namespace
             hookargs['key'] = key
             hookargs['old'] = old