diff -r 9183cb6886ef -r 5c35a6040352 mercurial/changegroup.py
--- a/mercurial/changegroup.py	Tue Apr 28 13:40:00 2015 -0700
+++ b/mercurial/changegroup.py	Tue Apr 28 13:49:19 2015 -0700
@@ -440,6 +440,7 @@
         if clnodes:
             repo.hook('outgoing', node=hex(clnodes[0]), source=source)
 
+    # The 'source' parameter is useful for extensions
     def generatefiles(self, changedfiles, linknodes, commonrevs, source):
         repo = self._repo
         progress = self._progress