Add an option to use the sha1collisiondetection library from the
submodule in sha1collisiondetection/ instead of in the copy in the
sha1dc/ directory.
This allows us to try out the submodule in sha1collisiondetection
without breaking the build for anyone who's not expecting them as we
work out any kinks.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>