diff -r 4dc5b51f38fe -r 049005de325e mercurial/localrepo.py
--- a/mercurial/localrepo.py	Sun Aug 30 13:58:11 2015 -0700
+++ b/mercurial/localrepo.py	Sun Aug 30 14:03:32 2015 -0700
@@ -354,6 +354,10 @@
         manifestcachesize = self.ui.configint('format', 'manifestcachesize')
         if manifestcachesize is not None:
             self.svfs.options['manifestcachesize'] = manifestcachesize
+        # experimental config: format.aggressivemergedeltas
+        aggressivemergedeltas = self.ui.configbool('format',
+            'aggressivemergedeltas', False)
+        self.svfs.options['aggressivemergedeltas'] = aggressivemergedeltas
 
     def _writerequirements(self):
         scmutil.writerequires(self.vfs, self.requirements)