Add completion to magit-vcsh-status

[travisbhartwell]

vcsh list gives a list of the repos that it knows about, so it would be awesome if magit-vcsh-status allowed for completion on those names, similar to how magit-status uses magit-read-repository to complete on the repositories it finds in magit-repository-directories. It would be simpler than magit-read-repository since the list is given for you.

I don't have time immediately or I would attempt a patch.

Thanks for this great library. It's been my main pain point of using vcsh of not being able to use magit with it.