dirstate: don't fail when dropping a not-tracked file (issue3080)

Complex merges with divergent renames can cause a file to be 'moved'
twice, causing dirstate.drop() to be called twice. Rather than try to
ensure there are no unexpected corner cases where this can happen, we
simply ignore drops of files that aren't tracked.

mercurial/dirstate.py

@@ -370,9 +370,10 b' class dirstate(object):'
 
     def drop(self, f):
         '''Drop a file from the dirstate'''
-        self._dirty = True
-        self._droppath(f)
-        del self._map[f]
+        if f in self._map:
+            self._dirty = True
+            self._droppath(f)
+            del self._map[f]
 
     def _normalize(self, path, isknown):
         normed = os.path.normcase(path)