Makefile: under "make dist", include the sha1collisiondetection submodule

Include the sha1collisiondetection submodule when running "make dist". Even though we've been shipping the sha1collisiondetection submodule[1] and using it by default if it's checked out[2] anyone downloading git as a tarball would just get an empty sha1collisiondetection/ directory. Doing this automatically is a feature that's missing from git-archive, but in the meantime let's bundle this up into the tarball we ship. This ensures that the DC_SHA1_SUBMODULE flag does what's intended even in an unpacked tarball, and more importantly means we're building the exact same code from the same paths from git.git and from the tarball. I am not including all the files in the submodule, only the ones git actually needs (and the licenses), only including some files like this would be a useful feature if git-archive ever adds the ability to bundle up submodules. 1. commit 86cfd61e6b ("sha1dc: optionally use sha1collisiondetection as a submodule", 2017-07-01) 2. cac87dc01d ("sha1collisiondetection: automatically enable when submodule is populated", 2017-07-01) Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>