Add API for getting at git_diff_patch->content_size

I'd love to be able to decide how to handle a large diff based on the content size being over a specific threshold. To do that I'd need to know the size of a patches contents and since we appear to already be keeping track of that I thought it'd be nice to expose it via something like `git_diff_patch_content_size(git_diff_patch *patch, size_t *content_size, size_t hunk_idx)`.

What do you think?