$repo->merge can't merge against a tag. (Solution code included)

[John-Schlick]

In general git can merge against a branch or a tag as long as it can translate that to a commit sha.
The current codebase insists that what it passed to merge be a branch.
I NEED to be able to merge against tags for the release process that we have.

So, the following code will allow you to pass a branch OR a tag to repository->merge
First remove the branch specifier from merge in repository.php:
public function merge($branch)

and then recode MergeCommand.php merge to look like:
public function merge($with)
{
$this->clearAll();
$this->addCommandName(static::MERGE_COMMAND);
// $with may not always be a branch. It can be a tag.
if (method_exists($with, 'setFullRef')) {
$ref = $with->getFullRef();
} else {
$ref = $with->getName();
}
$this->addCommandSubject($ref);

    return $this->getCommand();
}

This removes the Branch specifier from the calling parameters, and also checks to make sure that the getFullRef is there (when it's a branch, if not it assumes it's a tag and uses getName)

This is not a perfect implementation as we should also be able to pass in a commit, but since I haven't had to deal with that yet, I've done what I need, and this is healthier than it was.