transaction: pass the transaction to 'pending' callback

The callback will likely need to perform some operation related to the
transaction (eg: backing files up). So we better pass the current transaction as
the callback argument. Otherwise callback that needs it has to rely on horrible
weak reference trick.

The first foreseen user of this is changelog._writepending. We would like it to
register the temporary file it create for cleanup purpose.

mercurial/changelog.py

@@ -270,7 +270,7 b' class changelog(revlog.revlog):'
         self._nodecache = r._nodecache
         self._chunkcache = r._chunkcache
 
-    def _writepending(self):
+    def _writepending(self, tr):
         "create a file containing the unfinalized state for pretxnchangegroup"
         if self._delaybuf:
             # make a temporary copy of the index

mercurial/transaction.py

@@ -281,6 +281,8 b' class transaction(object):'
     def addpending(self, category, callback):
         """add a callback to be called when the transaction is pending
 
+        The transaction will be given as callback's first argument.
+
         Category is a unique identifier to allow overwriting an old callback
         with a newer callback.
         """
@@ -294,7 +296,7 b' class transaction(object):'
         categories = sorted(self._pendingcallback)
         for cat in categories:
             # remove callback since the data will have been flushed
-            any = self._pendingcallback.pop(cat)()
+            any = self._pendingcallback.pop(cat)(self)
             self._anypending = self._anypending or any
         return self._anypending