localrepo: prevent leak of transaction object (issue4258)

The onclose() closure added in cd443c7589cc held a regular reference to
the transaction object. This was causing the transaction to not gc and
a leak to occur.

The closure now holds a reference to the weakref instance and the leak
goes away.

mercurial/localrepo.py

@@ -858,7 +858,7 b' class localrepository(object):'
                 _("abandoned transaction found - run hg recover"))
 
         def onclose():
-            self.store.write(tr)
+            self.store.write(self._transref())
 
         self._writejournal(desc)
         renames = [(vfs, x, undoname(x)) for vfs, x in self._journalfiles()]