contrib: move "coverage-diff.sh" script into "tools/"

contrib: move "coverage-diff.sh" script into "tools/"

The "coverage-diff.sh" script can be used to get information about test
coverage fro the Git codebase. It is thus rather specific to our build
and test infrastructure and part of the developer-facing tooling. The
fact that this script is part of "contrib/" is thus rather misleading
and a historic wart.

Promote the tool into the new "tools/" directory.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>