[git-buildpackage] Deprecating Gbp-Pq-Topic

[Guido Günther]

Hi Markus,
I noticed that entries like

  Gbp-Pq: Topic <topic>

don't get filtered out of the resulting patch file while

  Gbp-Pq-Topic: <topic>

gets filtered out so the result of

  gbp pq export

is different depending on wether "Gbp-Pq-Topic:" or "Gbp-Pq: Topic"
got used. Is this intentinal? Both commands should produce the same
results. The current parser doesn't seem cappable of removing commands
from the commit message.
Cheers,
 -- Guido