merge: more safe detection of criss cross merge conflict between dm and r

41f6af50c0d8 introduced handling of a crash in this case. A review comment
suggested that it was not entirely obvious that a 'dm' always would have a 'r'
for the source file.

To mitigate that risk, make the code more conservative and make less
assumptions.

mercurial/merge.py

@@ -1039,8 +1039,7 b' def calculateupdates(repo, wctx, mctx, a'
         for f in dms:
             dm, (f0, flags), msg = actions[f]
             assert dm == 'dm', dm
-            m, args, msg = actions[f0]
-            if m == 'r':
+            if f0 in actions and actions[f0][0] == 'r':
                 # We have one bid for removing a file and another for moving it.
                 # These two could be merged as first move and then delete ...
                 # but instead drop moving and just delete.