Test for tracking deleted branch

Got a question over email:

> we just cleaned up our origin and stripped it of a bunch of branches that are no longer used.  If the user’s local repo is set to one of those cleaned branches, I fail out when I try to get the ahead/behind status of the branch.  I would like to be able to automatically prune local of these deprecated branches, but I can’t find a way to use LibGit2Sharp to accomplish that.

The attached test simply proves (I think) that we're doing The Right Thing™ for branches tracking pruned remote refs.