test: use the internal `_normallookup` in `test-rebuildstate.t`

This is a low level test doing low level manipulation.

Differential Revision: https://phab.mercurial-scm.org/D11180

tests/test-rebuildstate.t

@@ -17,7 +17,7 b''
   >   try:
   >     for file in pats:
   >       if opts.get('normal_lookup'):
-  >         repo.dirstate.normallookup(file)
+  >         repo.dirstate._normallookup(file)
   >       else:
   >         repo.dirstate._drop(file)
   >