py3: avoid changing dictionary during iteration

dict.items() and friends are iterators/views in Python 3. You
aren't allowed to mutate the underlying dictionary when iterating
on these views. So iterate over a copy of things.

Differential Revision: https://phab.mercurial-scm.org/D2164

mercurial/copies.py

@@ -123,7 +123,7 b' def _chain(src, dst, a, b):'
             t[k] = v
 
     # remove criss-crossed copies
-    for k, v in t.items():
+    for k, v in list(t.items()):
         if k in src and v in dst:
             del t[k]