dirstate: remove file from copymap on drop

As the copymap is short-lived object regenerated from dirstate on each
read this didn't affect us in any serious way. But since I've started working
on permanent storage of copymap in my experiments with sqldirstate[1] I've seen
this bug leaving the copy information in copymap after reverting the file
moves and copies.

[1] https://www.mercurial-scm.org/wiki/SQLDirstatePlan

mercurial/dirstate.py

@@ -582,6 +582,8 b' class dirstate(object):'
             del self._map[f]
             if f in self._nonnormalset:
                 self._nonnormalset.remove(f)
+            if f in self._copymap:
+                del self._copymap[f]
 
     def _discoverpath(self, path, normed, ignoremissing, exists, storemap):
         if exists is None: