Bug #915629: Allow --ignore-branch for 'gbp pq'

[j1elo]

• 4c0f956 adds support for the import command. Is this a good way to do it?

• 22fe91b is a cosmetic change just to generate shorter patch-queue/ branch names. With this, patch queue branches will be named such as patch-queue/03e0b41. Without this, pq branches would be named like patch-queue/03e0b4134690062fb7a358592f85df8c7ff31bbd. This is just a cosmetic commit, not very important given that patch-queue branches tend to be short lived. But it's added here just in case it's seen as a good idea.