Automatically fixup linked issues in subtree repository

[Urgau]

This PR adds a new handler to "relink" linked GitHub issues into their unambiguous form so that when pulling commits from a subtree into the main repository we don't accidentally closes issues in the wrong repository.

Example: Fixes #123 (in rust-lang/clippy) would be automatically changed to Fixes rust-lang/clippy#123

r? @ehuss

[jackh726]

I think we should gate this behind a manual command to do the relinking, in case there are issues we don't wreck everyone's PR descriptions.

[Urgau]

It's already gated behind the [relink] config, if there are any issue (which is very unlikely IMO) the relink can be disabled by removing the section from the configuration file.

Putting the relink behind a manual command renders (IMO) this handler/pass useless, as I don't think anyone is going to use it and the situation is going to stay the same without any improvements.

If we are worried about edge-cases we might miss, we can do a slow deployment, starting with lower traffic repos, and go higher trafic repo as we have more confidence in it.

[jackh726]

Hmm, I suppose it's fine to land this without a gate if we start with a slow repo and allow plenty of testing time