merge: fix test failures with new merge code on OS X

The case collision checker was missing a op entry for 'k'eep.

mercurial/merge.py

@@ -353,6 +353,7 b' def _checkcollision(repo, wmf, actions):'
         "dg": renamegetop,
         "dr": nop,
         "e": nop,
+        "k": nop,
         "f": addop, # untracked file should be kept in working directory
         "g": addop,
         "m": mergeop,