tests/test-abort-checkin.t

commit: warn the user when a commit already exists

Sometimes, a commit will result in an exact match of a preexisting
commit, and if that commit isn't a branch head, hg will incorrectly
note that it created a new head. Instead, we should warn the user that
commit already existed in the repository.

In practice, this bug is rather uncommon, and will only occur when the
usr explicitly sets the date.

Please note that this commit contains an API change to
cmdutil.commitstatus()

Differential Revision: https://phab.mercurial-scm.org/D9257

  $ cat > abortcommit.py <<EOF
  > from mercurial import error
  > def hook(**args):
  >     raise error.Abort(b"no commits allowed")
  > def reposetup(ui, repo):
  >     repo.ui.setconfig(b"hooks", b"pretxncommit.nocommits", hook)
  > EOF
  $ abspath=`pwd`/abortcommit.py

  $ cat <<EOF >> $HGRCPATH
  > [extensions]
  > mq =
  > abortcommit = $abspath
  > EOF

  $ 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 ..