This was pulled over from git.git, and is an experiment in
making binary-searching lists of sha1s faster. It was never
compiled by default (nor was it used upstream by default
without a special environment variable).

Unfortunately, it is actually slower in practice, and
upstream is planning to drop it in
git/git@f1068efefe6dd3beaa89484db5e2db730b094e0b (which has
some timing results). It's worth doing the same here for
simplicity.