sha1collisiondetection: automatically enable when submodule is populated

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>