config: use the new '_unset' value for 'configbool'
This should let 'configbool' delegate all special processing of the default
config value to the main 'config' method.
The default value for bool (False) is still enforced in this method if no other
default were passed.
$ cat > abortcommit.py <<EOF
> from mercurial import error
> def hook(**args):
> raise error.Abort("no commits allowed")
> def reposetup(ui, repo):
> repo.ui.setconfig("hooks", "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 ..