mrhlpr: use Part-of trailer instead of adding MR ID to subject

We are planning to migrate to marge-bot for handling merging commits, and with it adopt a new way of linking commits to their source MRs.

Using a trailer with Part-of: !MR-ID results in the commits being linked to the MR, and the GitLab web interface inserting a hyperlink in its place making it easier to jump to the source MR.

Let's adopt this for MRs we still merge manually now.

Use the full URL to the MR rather than just the ID, this way if we ever split or merge repos it will still be easily possible to trace commits back to their original MRs.

Signed-off-by: Caleb Connolly caleb@postmarketos.org