Add helper API for diff line stats from patch

This adds a `git_diff_patch_line_stats()` API that gets the total number of adds, deletes, and context lines in a patch.  This will make it a little easier to emulate `git diff --stat` and the like.

Right now, this relies on generating the `git_diff_patch` object, which is a pretty heavyweight way to get stat information.  At some future point, it would probably be nice to be able to get this information without allocating the entire `git_diff_patch`, but that's a much larger project.