$repo->checkout can't specify amongst multiple remote branches. (Solution code included)

[John-Schlick]

Please see the article at:
http://makandracards.com/makandra/14323-git-how-to-check-out-branches-that-exist-on-multiple-remotes

It talks about how to specify amongst remote branches at checkout. Since you also dont support git checkout -b thats included as it's necessary to get teh right syntax.

If you change mainCommand->checkout to look like this:
public function checkout($ref, $createBranch = false, $refSource=null)
{
$this->clearAll();

    $what = $ref;
    if ($ref instanceof Branch) {
        $what = $ref->getName();
    } elseif ($ref instanceof TreeishInterface) {
        $what = $ref->getSha();
    }

    $this->addCommandName(self::GIT_CHECKOUT);
    $this->addCommandArgument('-q');
    if ($createBranch) {
        $this->addCommandArgument('-b');
    }
    $this->addCommandSubject($what);
    if (!is_null($refSource)) {
        $this->addCommandSubject2($refSource);
    }
    return $this->getCommand();
}

and then change repository->checkout to look like:
public function checkout($ref, $createBranch = false, $refSource=null)
{
$this->caller->execute(MainCommand::getInstance()->checkout($ref, $createBranch, $refSource));

    return $this;
}

This will allow a caller to specify a SPECIFIC source for their checkout (if the local branch does not already exist.), as well as specifying that they want a branch created.