support `git swtich`

[choang]

Maybe this is a local problem for me, but here's something I just did and could see myself doing more of:

• git fetch origin somebranch:somebranch
• git switch somebranch
• gs btr somebranch

It would be great if I could do this in git-spice. Note, I had to fetch that branch specifically instead of a general because I also have refspec filters in my .gitconfig.

[abhinav]

AFAIK, git switch is just a better named alternative to git checkout $branch?
Could you clarify which part of the behavior you're thinking of supporting?

• check out and auto-track the branch
• fetch, check out, and auto-track a branch