sno switch [branch] doesn't switch to upstream branch if it exists

[hamishcampbell]

Given a remote origin with a branch other-branch, the following command does not currently work as expected:

$ sno switch other-branch
Error: Branch 'other-branch' not found.

To check out a remote branch and configure tracking, it seems you currently have to do something like this:

$ sno switch origin/other-branch
$ sno switch -c other-branch
$ sno push --set-upstream origin other-branch

I expected sno switch other-branch do to this for me. That is:

1. Look for a remote branch with the name other-branch
2. check that branch out
3. configure the upstream source

[rcoup]

👍 needs to match git's behaviour