copies: move code into new manifestdict.filesnotin() method

copies._computeforwardmissing() finds files in one context that is not
in the other. Let's move this code into a new method on manifestdict,
so m1.filesnotin(m2) can be optimized for various types of manifests
(we expect more types of manifests soon).

mercurial/copies.py

@@ -149,9 +149,7 b' 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'''

mercurial/manifest.py

@@ -38,6 +38,12 b' 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():