contrib: move "update-unicode.sh" script into "tools/"

The "update-unicode.sh" script is used to update the unicode data compiled into Git whenever a new version of the Unicode standard has been released. As such, it is a natural part of our developer-facing tooling, and its presence in "contrib/" is misleading. Promote the script into the new "tools/" directory. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>