$ 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

  [255]

  $ hg ci -m foo

  error: pretxncommit.nocommits hook failed: no commits allowed

  transaction abort!

  rollback completed

  abort: no commits allowed

  [255]

  $ cd ..