If a user wants to experiment with the version of collision
detecting sha1 from the submodule, the user needed to not just
populate the submodule but also needed to turn the knob.
A Makefile trick is easy enough to do so, so let's do this. When
somebody with a copy of the submodule populated wants not to use it,
that can be done by overriding it in config.mak or from the command
line.
Signed-off-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>