When using "-p" and "-b" in combination with "-u", the output is not
what you would expect. The reason is the sd_journal_add_disjunction()
call in add_matches_for_unit() and add_matches_for_user_unit(), which
adds two ORs without taking the other conditions to every OR.

Adding another level on top with AND and sd_journal_add_conjunction()
solves the problem.

Output before:

$ journalctl -o short-monotonic -ab -p 0 -u sshd.service

-- Reboot --
[    3.216305] lenovo systemd[1]: Starting OpenSSH server daemon...
-- Reboot --
[    3.168666] lenovo systemd[1]: Starting OpenSSH server daemon...
[    3.169639] lenovo systemd[1]: Started OpenSSH server daemon.
[36285.635389] lenovo systemd[1]: Stopped OpenSSH server daemon.
-- Reboot --
[   10.838657] lenovo systemd[1]: Starting OpenSSH server daemon...
[   10.913698] lenovo systemd[1]: Started OpenSSH server daemon.
[ 6881.035183] lenovo systemd[1]: Stopped OpenSSH server daemon.
-- Reboot --
[    6.636228] lenovo systemd[1]: Starting OpenSSH server daemon...
[    6.662573] lenovo systemd[1]: Started OpenSSH server daemon.
[    6.681148] lenovo sshd[397]: Server listening on 0.0.0.0 port 22.
[    6.681379] lenovo sshd[397]: Server listening on :: port 22.

As we see, the output is from _every_ boot and priority 0 is not taken
into account.

Output after patch:

$ journalctl -o short-monotonic -ab -p 0 -u sshd.service
-- Logs begin at Sun 2013-02-24 20:54:44 CET, end at Tue 2013-03-19 14:58:21 CET. --

Increasing the priority:

$ journalctl -o short-monotonic -ab -p 6 -u sshd.service
-- Logs begin at Sun 2013-02-24 20:54:44 CET, end at Tue 2013-03-19 14:59:12 CET. --
[    6.636228] lenovo systemd[1]: Starting OpenSSH server daemon...
[    6.662573] lenovo systemd[1]: Started OpenSSH server daemon.
[    6.681148] lenovo sshd[397]: Server listening on 0.0.0.0 port 22.
[    6.681379] lenovo sshd[397]: Server listening on :: port 22.