diff --git a/mercurial/copies.py b/mercurial/copies.py
--- a/mercurial/copies.py
+++ b/mercurial/copies.py
@@ -149,9 +149,7 @@ def _computeforwardmissing(a, b):
     This is its own function so extensions can easily wrap this call to see what
     files _forwardcopies is about to process.
     """
-    missing = set(b.manifest().iterkeys())
-    missing.difference_update(a.manifest().iterkeys())
-    return missing
+    return b.manifest().filesnotin(a.manifest())
 
 def _forwardcopies(a, b):
     '''find {dst@b: src@a} copy mapping where a is an ancestor of b'''
diff --git a/mercurial/manifest.py b/mercurial/manifest.py
--- a/mercurial/manifest.py
+++ b/mercurial/manifest.py
@@ -38,6 +38,12 @@ class manifestdict(dict):
                     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():