obsmarker: record the number of new markers in the transaction

This lets hooks be notified about new markers in the repository.

mercurial/obsolete.py

@@ -400,6 +400,9 b' class obsstore(object):'
             self._load(new)
             # new marker *may* have changed several set. invalidate the cache.
             self.caches.clear()
+        # records the number of new markers for the transaction hooks
+        previous = int(transaction.hookargs.get('new_obsmarkers', '0'))
+        transaction.hookargs['new_obsmarkers'] = str(previous + len(new))
         return len(new)
 
     def mergemarkers(self, transaction, data):