Revert changes from git/git diff-delta.c by dak@gnu.org, proski@gnu.org

The following commits to git/git `diff-delta.c` by David Kastrup and Pavel Roskin have not yet been relicensed under our license, the GPL v2 with Linking Exception. We should not redistribute their work with libgit2 until we get such permission from the authors.

Accordingly, this change splices out the following commits:

git/git@02e665ce491296245f474dafdc02d47a6c8afa86
git/git@d2100860fd67dec6474157697888caaa0a0f51d0
git/git@b1d884a9e3968db1fff91c2d066d871a3b8b013c
git/git@82e5a82fd73edb80a841f5fab1660e14b9b8f3ad

This change also puts in place the standard header for libgit2 at the top of `delta.c`.