repo: Clean up temporaries after a transaction completes

Prevously, we were just leaving temporary files there forever if
a transaction was interrupted.

https://bugzilla.gnome.org/show_bug.cgi?id=706344
repo: Clean up tmpdir also on transaction abort

Pull the cleanup code to a helper function, and ensure we delete
leftover temporary files also when aborting a transaction.  Mainly
this will happen if a local 'ostree commit' fails.

While we're here, also change it to use gs_shutil_rm_rf() which also
handles directories, should we start using those.

Reviewed-by: Jeremy Whiting <jpwhiting@kde.org>