Enable git diff-tree with empty second tree-ish param

[gangefors]

Executing git diff-tree and omitting the second tree-ish argument
wasn't possible before. This PR enables this behaviour by allowing
the caller to send in an empty string.

Allowing an empty string keeps backwards compatibility for all other
allowed input types.

[gangefors]

It might be that the thing I'm trying to achieve here is the same result I get if I use git.diff.NULL_TREE. Feel free to ignore and close if that is the case.

[Byron]

@gangefors Thanks for your effort :)! It’s hard for me to tell if git.diff.NULL_TREE would have the same result, just because I am not on a ‘real’ computer right now. However, if that was the case the test provided here would work similarly, which is something you could check.

If it’s not the same, I would prefer to see the empty string become some constant, like git.diff.PARENT_TREE.

[Byron]

Thanks a lot for your contribution thus far !
I am closing this PR due to prolonged inactivity. Please feel free to reopen it in case it can proceed.