Start implementing "git-apply"

Start implementing "git-apply"

This applies git patches (and old-style unified diffs)
in the index, rather than doing it in the working directory.

That allows for a lot more flexibility, and means that if a
patch fails, we aren't going to mess up the working directory.

NOTE! This is just the first cut at it, and right now it only
parses the incoming patch, it doesn't actually apply it yet.