patch: copy copies dict before changing it (issue1651)

mercurial/patch.py

@@ -1236,6 +1236,7 b' def diff(repo, node1=None, node2=None, m'
 
     if opts.git:
         copy, diverge = copies.copies(repo, ctx1, ctx2, repo[nullid])
+        copy = copy.copy()
         for k, v in copy.items():
             copy[v] = k