manifest: improve filesnotin performance by using lazymanifest diff

lazymanifests can compute diffs significantly faster than taking the set
of two manifests and calculating the delta.

when running hg diff --git -c . on Facebook's big repo, this reduces the
run time from 2.1s to 1.5s.

mercurial/manifest.py

@@ -211,8 +211,10 b' class manifestdict(object):'
 
     def filesnotin(self, m2):
         '''Set of files in this manifest that are not in the other'''
-        files = set(self)
-        files.difference_update(m2)
+        diff = self.diff(m2)
+        files = set(filepath
+                    for filepath, hashflags in diff.iteritems()
+                    if hashflags[1][0] is None)
         return files
 
     @propertycache