signer: Check that PRs will go to the right place

[jku]

It would be trivial to check if any git remote is theupdateframework/tuf-on-ci-template when signer starts: this is some sort of forking/cloning error and we could error out with a message explaining that.

cc @sorayaormazabalmayo

[jku]

Hmm, I think this was not the actual issue that lead to filing this: the real problem was that the docs implied forking the template repo is correct, when "Use This Template" is actually correct. If a real fork was used, then PRs would get directed towards tuf-on-ci-template by GitHub -- this is not ideal.

I think we can't really figure this issue from signer config alone: it is visible in GitHub API though and could at least theoretically be checked in signer:

• In signer we can check if pull-remote and push-remote are the same and
• we can parse ORG and REPO for the remote (I think this is already done at least in one place):
• if the rest API response for repos/ORG/REPO contains a "parent" value, then the repo is a fork and and the suggested PR will go to the parent

This sounds a little too complicated though