merge: add removefile() to mergeresult object

There are cases where some further calculation makes the file not needing to be
merged anymore and hence needs to be dropped in mergeresult object. This adds a
function for that.

Differential Revision: https://phab.mercurial-scm.org/D8821

mercurial/merge.py

@@ -576,6 +576,11 b' class mergeresult(object):'
         """
         self._actions[filename] = (action, data, message)
 
+    def removefile(self, filename):
+        """ removes a file from the mergeresult object as the file might
+        not merging anymore """
+        del self._actions[filename]
+
     @property
     def actions(self):
         return self._actions
@@ -1929,7 +1934,7 b' def update('
                         b'prompt recreating',
                     )
                 else:
-                    del mresult.actions[f]
+                    mresult.removefile(f)
 
         # Convert to dictionary-of-lists format
         actions = mresult.actionsdict