Add Jonathan Tan to git.git-authors

Jonathan has consented via email to have his contributions to git reused in libgit2

Author: Charlie Somerville

git.git-authors

@@ -53,6 +53,7 @@ ok	Jeff King <peff@peff.net>
 ok	Johannes Schindelin <Johannes.Schindelin@gmx.de>
 ok	Johannes Sixt <j6t@kdbg.org>
 ask	Jonathan Nieder <jrnieder@gmail.com>
+ok	Jonathan Tan <jonathantanmy@google.com>
 ok	Junio C Hamano <gitster@pobox.com>
 ok	Kristian Høgsberg <krh@redhat.com>
 ok	Linus Torvalds <torvalds@linux-foundation.org>