copies: calculate 'bothnew' from manifestdict.filesnotin()

In the same spirit as the previous change, let's now calculate the
'bothnew' variable using manifestdict.filesnotin().5D

mercurial/copies.py

@@ -302,7 +302,9 b' def mergecopies(repo, c1, c2, ca):'
         else:
             diverge2.update(fl) # reverse map for below
 
-    bothnew = sorted([d for d in m1 if d in m2 and d not in ma])
+    addedinm1 = m1.filesnotin(ma)
+    addedinm2 = m2.filesnotin(ma)
+    bothnew = sorted(addedinm1 & addedinm2)
     if bothnew:
         repo.ui.debug("  unmatched files new in both:\n   %s\n"
                       % "\n   ".join(bothnew))