merge: pconvert paths in _unknowndirschecker before dirstate-normalizing

This fixes the failure in test-pathconflicts-basic.t on Windows.  The test was
passing in 'a\b', which was getting normalized to 'A\B', which isn't in
dirstate.  (The filesystem path is all lowercase anyway.)

This isn't the only case of calling dirstate.normalize(), but other methods here
(util.finddirs()) seem to assume the input paths are already using '/'.  I think
the backslash comes from wvfs.reljoin() (in this case), but could also come from
wvfs.walk(), so this is the only case that needs it.

mercurial/merge.py

@@ -706,7 +706,8 b' class _unknowndirschecker(object):'
             # Does the directory contain any files that are not in the dirstate?
             for p, dirs, files in repo.wvfs.walk(f):
                 for fn in files:
-                    relf = repo.dirstate.normalize(repo.wvfs.reljoin(p, fn))
+                    relf = util.pconvert(repo.wvfs.reljoin(p, fn))
+                    relf = repo.dirstate.normalize(relf)
                     if relf not in repo.dirstate:
                         return f
         return None