Remove all the magic from overloading.  They're just object ID strings.

Since there's several different ways to sort commits, and there's no sense to
sorting trees or blobs, remove all the extra overloading magic and just leave
them as object ID strings.

See https://github.com/jacquesg/p5-Git-Raw/pull/152#issuecomment-60644808
Merge pull request #152 from schwern/issue/151

Add fallback to all the overloads.