cli: add --create to branch set to upsert branches

[Cretezy]

Fixes #3584. Awaiting final decision on approach in the issue to remove draft status.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have added tests to cover my changes

[khionu]

What's the purpose of this? branch create can already take -r

Found #3584, advocated for --allow-new over --create for mindset reasons.

[jonathantanmy]

Looks good to me - the diff is shorter than I expected, but that's because set_local_branch_target already performs an upsert. I haven't contributed to jj in a while so I'll let someone else approve (in particular, I can't tell if this particular --create argument is the way the project wants to go, or if we should use another name).

[yuja]

Found #3584, advocated for --allow-new over --create for mindset reasons.

I also like --allow-new or --allow-create because this option is noop if the branch already exists.

It might be also good to rename the current typo-safe version and add new create-or-update "set", but I'm not sure.
#3584 (comment)