--- a/mercurial/manifest.py	Fri Feb 27 23:30:42 2015 -0500
+++ b/mercurial/manifest.py	Fri Feb 27 13:57:37 2015 -0800
@@ -38,6 +38,12 @@
                     ret._flags[fn] = flags
         return ret
 
+    def filesnotin(self, m2):
+        '''Set of files in this manifest that are not in the other'''
+        files = set(self.iterkeys())
+        files.difference_update(m2.iterkeys())
+        return files
+
     def matches(self, match):
         '''generate a new manifest filtered by the match argument'''
         if match.always():