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>

Author: pks-t