Do pre-merge checks on commands during incoming processing

There are various things that can be wrong in a merge request: the target branch might not be managed by pqm, the submitter might not have permission to commit to it, or the commit regex might not match the requested commit message.

PQM should check all these things (which can be done without needing temporary disk resources or other things we should lock for) before adding the request to the queue, giving the submitter much faster feedback.

This would be quite a good usability improvement.