t2u and dgit's keyid determination

In my tests, to make progress, I had to bodge this argument onto drs's dgit invocation:

	 -kdebian-tag2upload@lists.debian.org

I looked at the code in dgit and there doesn't seem to be a way to say "don't pass -u to gnupg". It looks at the -k option, or keyid from the access cfg, or Maintainer from the changelog. I think this is wrong and I'll make an MR to fix it but can't easily be fixed.

With that MR it will be possbile to have pass dgit -k '' to mean "don't pass a keyid option". That will make would have made it use "whatever key is available" like the comment in dgit-repos-server says. There's a TODO there, though, saying that this isn't ideal.