dirstate: use the right internal API in a test script

The goal of the script it to perform arbitrary internal operation to create
incorrect state, so lets make it clear.

Differential Revision: https://phab.mercurial-scm.org/D11031

tests/test-rebuildstate.t

@@ -19,7 +19,7 b''
   >       if opts.get('normal_lookup'):
   >         repo.dirstate.normallookup(file)
   >       else:
-  >         repo.dirstate.drop(file)
+  >         repo.dirstate._drop(file)
   > 
   >     repo.dirstate.write(repo.currenttransaction())
   >   finally: