Source and License Information for xdiff Implementation in Git

1st chunk of `src/xdiff/README.txt`

f1bc672d4de910630fd0a5443b65e53ee8514259e95f37b1000000010000029e

The files in this directory come from the xdiff implementation in git.
You can find it here: https://github.com/git/git/tree/master/xdiff
The files were last updated August 31, 2021 from git release v.2.33.0

This is originally based on libxdiff, which can be found here:
http://www.xmailserver.org/xdiff-lib.html

The git version was used because it has been maintained and improved.
And since it's part of git it is expected to be reliable.

The code is distributed under the GNU LGPL license.  It is included in the
COPYING file.

Changes in these files were made to avoid compiler warnings.

The first work for including xdiff in Vim was done by Christian Brabandt.

Title: Source and License Information for xdiff Implementation in Git

Summary

This text describes the origin of the xdiff implementation used in the project. It clarifies that the code is derived from the xdiff implementation in Git (originally based on libxdiff), specifies the version used, highlights its reliability due to being part of Git, mentions the GNU LGPL license under which it's distributed, notes changes made to avoid compiler warnings, and acknowledges Christian Brabandt's initial work on integrating xdiff in Vim.