dirstate: drop workaround for '.' matching root directory

The check was added in 31abcae33b4f (dirstate: do not ignore current
directory '.' (issue 1078), 2008-04-05) to fix issue1078. Funnily
enough, comment  #2 on that issue mentions using '' instead of '.' to
represent the root directory, just like my previous patch did.

test-hgignore.t fails with this patch without the previous patch.

Differential Revision: https://phab.mercurial-scm.org/D6402

mercurial/dirstate.py

@@ -662,8 +662,6 b' class dirstate(object):'
         self._dirty = False
 
     def _dirignore(self, f):
-        if f == '.':
-            return False
         if self._ignore(f):
             return True
         for p in util.finddirs(f):