Implement command-line options to change `branch.<name>.remote/pushRemote` configs

[Denton-L]

Perhaps something like --set-remote-to, --set-push-remote-to, --set-remote-save-to-push?

For the last, this would behave similarly to f39a9c6 (remote: add --save-to-push option to git remote set-url, 2018-12-10).

That option is useful for a situation such as mirroring. Consider if I wanted to mirror an upstream branch on my own remote. I would have something such as

[branch "master"]
    remote = "upstream"
    pushRemote = "origin"

in my config.

We could make this efficient to setup by doing:

git checkout master # is currently tracking origin's master
git branch --set-remote-save-to-push upstream

This is already implemented here but I think this idea might be a little too stupid to contribute. Maybe I'll send it in one day...

[Denton-L]

Probably too obscure to be worth a command-line option.