tags: don't check for clean files when checking for dirty working copy
We didn't request status for clean files so there should never be any
entries in the list of clean files (at least not since dd773340a085),
but it's misleading to check the list.
Differential Revision: https://phab.mercurial-scm.org/D7428
#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
Ran 1 tests, 1 results as expected (*) (glob)