Ask Git to track remote + branch when auto-pulling PR's (--set-upstream)

[holta]

To help with iiab-summary and iiab-diagnostics.

As discussed at:

• https://github.com/iiab/iiab/pull/3173#issuecomment-1175598922

Alongside similar efforts like:

• PR iiab/iiab#3270
• PR iiab/iiab#3280
• PR #221
• PR #224

[holta]

This PR is really overkill, given that devs not uncommonly want to push/pull to their own upstream (rather than the PR's original upstream).

Still, let's give this a shot, to see how this goes.

(Consider this a hack for now. That we can revert later if there's a better way, which there might possibly be in future.)

And just a reminder that git pull --set-upstream does little more than set both these variables in .git/config :

branch.<actual-branch-name>.remote
branch.<actual-branch-name>.merge