dirstate: fix filefoldmap incosistency on file delete

The _filefoldmap is not updated in when files are deleted from dirstate. In the
case where the file with the same but differently cased name is added afterwards
it renders _filefoldmap incorrect.  Those steps must occur to for a problem to
reproduce:
 - call status (with listunknown=True),
 - update working rectory to a commit which does a casefolding change (A -> a)
 - call status again (it will show the file "a" as deleted)

Unfortunately I'm unable to write a test for it because I don't know any
core-mercurial command able to reproduce those steps.

The bug was originally spotted when hgwatchman was enabled. It caused the
changeset contents change during hg rebase (one file unrelarted to changeset
was deleted in it after rebase).

The hgwatchman is able to hit it because when hgignore changes the hgwatchmans
overridestatus is calling original status with listunknown=True.

mercurial/dirstate.py

@@ -445,6 +445,11 b' class dirstate(object):'
         if self[f] not in "?r" and "_dirs" in self.__dict__:
             self._dirs.delpath(f)
 
+        if "_filefoldmap" in self.__dict__:
+            normed = util.normcase(f)
+            if normed in self._filefoldmap:
+                del self._filefoldmap[normed]
+
     def _addpath(self, f, state, mode, size, mtime):
         oldstate = self[f]
         if state == 'a' or oldstate == 'r':