transaction: add a way to know a transaction has been finalized

This will be useful to fix the timing of the branchmap on disk caching.

Differential Revision: https://phab.mercurial-scm.org/D12127

mercurial/transaction.py

@@ -229,6 +229,10 b' class transaction(util.transactional):'
         if self._journal:
             self._abort()
 
+    @property
+    def finalized(self):
+        return self._finalizecallback is None
+
     @active
     def startgroup(self):
         """delay registration of file entry