Hello all!

I've attached a basic patch that adds an option to vc-working-revision. The
option is named *concrete* and if non-nil, it forces vc-working-revision to
return a revision name that will not go stale after new revisions are made.

This is useful for, e.g. git, where vc-working-revision will just return
the branch name, which only refers to the current commit for as long as
it's the head of the branch.

I'm using this in diff-hl #33 <https://github.com/dgutov/diff-hl/issues/33>to
determine when to refresh the current VC highlighting.

I've supplied an implementation for Git, and no-op implementations for all
the other backends. For most systems (i.e. all the other VCS systems I
know), the value of *concrete *does not matter. If you know a backend that
would benefit from a real implementation, please let me know.

Also, this is my first patch, so I'm not entirely sure I've got all my
ducks in a row. Any comments on that would be great too.

Thanks,
Jonathan