I don't know how things like this are generally handled in Debian, but would like to note that systemctl completion is not shipped by bash-completion (at least upstream) so chances are that it should be fixed somewhere else.
- Bug#808478: systemctl fails to autocomplete without dbus Joonas Kylmälä
- Bug#808478: [Bash-completion-devel] Bug#808478: syst... Ville Skyttä
- Bug#808478: [Bash-completion-devel] Bug#808478: ... Joonas Kylmälä

