patch 9.0.1418: the included xdiff code is a bit outdated

Problem:    The included xdiff code is a bit outdated.
Solution:   Sync with the latest git xdiff code. (Yee Cheng Chin,
            closes #12181)
diff --git a/src/xdiff/README.txt b/src/xdiff/README.txt
index 9e01a3c..3302abc 100644
--- a/src/xdiff/README.txt
+++ b/src/xdiff/README.txt
@@ -1,6 +1,6 @@
 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
+The files were last updated March 17, 2023 from git release v.2.40.0
 
 This is originally based on libxdiff, which can be found here:
 http://www.xmailserver.org/xdiff-lib.html