Suggest better branch name when reviewing PRs

[kaste]

If I just hit enter enough, I end up with e.g.

T.i. obviously the branch name has the remote name duplicated. What we probably want is also a backlink to the PR (a clickable link would be 👍, at least the number 🙏)

• # signs are not forbidden, t.i. master-#310 would be allowed, though weird. (#123-master is not allowed.)

So we have either 310-master or e.g. master-310 or master-#310. master is the branch name the contributor used; these are very often nonsensical, like master, patch-1 etc. (patch-1 would become patch-1-#23, ugly but useful.)