Add -v/--version support to rust binary

[Seldaek]

I assume this might not be accepted as is because it's not really using the full blown optgroups and whatnot but it's all I managed to get done so far and it still solves a frustrating issue I hit so I'll take my chances.

[bstrie]

I'm fine with this behavior because git has basically the same thing with git --version. And I don't expect it to be done with the official option-parsing lib because IIRC that's due for a rewrite anyway.

[thestinger]

Landing with #7490.