transaction: allow finalizer to add finalizer

It will make some code (persistent nodemap related) simpler to write, because
higher level code can blindly queue finalization without thinking too hard about
the context.

Differential Revision: https://phab.mercurial-scm.org/D7833

mercurial/transaction.py

@@ -506,9 +506,12 b' class transaction(util.transactional):'
             self._validator(self)  # will raise exception if needed
             self._validator = None  # Help prevent cycles.
             self._generatefiles(group=gengroupprefinalize)
-            categories = sorted(self._finalizecallback)
+            while self._finalizecallback:
+                callbacks = self._finalizecallback
+                self._finalizecallback = {}
+                categories = sorted(callbacks)
             for cat in categories:
-                self._finalizecallback[cat](self)
+                    callbacks[cat](self)
             # Prevent double usage and help clear cycles.
             self._finalizecallback = None
             self._generatefiles(group=gengrouppostfinalize)