Design inspired heavily by https://github.com/bradfitz/gitbrute .

'git hash-object ...' suggestion from rdebath in
bradfitz#7 .

Like bradfitz#5 , this program walks the
commit and authorship dates backward, rather than the current date.

Like bradfitz#8 , we have the same
problem.  Fortunately, current dates are 10 digits long; it would take about
2^58 iterations to actually come up with a shorter time_t in decimal; we're
way too CPU limited to realistically be brute forcing hashes with 58 bits of
prefix.