git: upgrade to 2.39.3

This minor point release fixes CVE-2023-25652 and CVE-2023-29007.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 469c2892)
Signed-off-by: Steve Sakoman <steve@sakoman.com>