tests/test-contrib-emacs.t

dirstate: enforce context set_clean and set_possibly_dirty

We don't want them called within a `changing_parents` context, but we still want
them called within a context.

So we update the decorator accordingly

#require emacs
  $ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
  >  -f ert-run-tests-batch-and-exit
  Running 1 tests (*) (glob)
     passed  1/1  hg-test-mode--compilation-mode-support* (glob)
  
  Ran 1 tests, 1 results as expected* (glob)