copies: eliminate criss-crosses when chaining

Before the copies refactoring, we declared that if a and b were
present in source and destination, we ignored copies between them. The
refactored code could however report b was a copy of a and vice versa
in a situation where we looked for differences between two identical
changesets that copy a to b.

  y
 /
x
 \
  y'

mercurial/copies.py

@@ -99,6 +99,13 @@ def _chain(src, dst, a, b):
         if v in src:
             # file is a copy of an existing file
             t[k] = v
+
+    # remove criss-crossed copies
+    for k, v in t.items():
+        if k in src and v in dst:
+            print "bad", k, v
+            del t[k]
+
     return t
 
 def _tracefile(fctx, actx):