manifest: correct the one use of iterkeys() on a dict

As far as I can tell, this is the only iterkeys() in this file that
was actually on a dict. The rest are on custom manifest types.

Differential Revision: https://phab.mercurial-scm.org/D2316

mercurial/manifest.py

@@ -960,7 +960,7 b' class treemanifest(object):'
                 else:
                     files.update(m1.iterkeys())
 
-            for fn in t1._files.iterkeys():
+            for fn in t1._files:
                 if fn not in t2._files:
                     files.add(t1._subpath(fn))