support `git swtich`

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.