$ cat > abortcommit.py <<EOF
> from mercurial import util
> def hook(**args):
> raise util.Abort("no commits allowed")
> def reposetup(ui, repo):
> repo.ui.setconfig("hooks", "pretxncommit.nocommits", hook)
> EOF
$ abspath=`pwd`/abortcommit.py
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ echo "abortcommit = $abspath" >> $HGRCPATH
$ hg init foo
$ cd foo
$ echo foo > foo
$ hg add foo
mq may keep a reference to the repository so __del__ will not be
called and .hg/journal.dirstate will not be deleted:
$ hg ci -m foo
error: pretxncommit.nocommits hook failed: no commits allowed
transaction abort!
rollback completed
abort: no commits allowed
$ hg ci -m foo
error: pretxncommit.nocommits hook failed: no commits allowed
transaction abort!
rollback completed
abort: no commits allowed
$ exit 0