This adds --bisect-vars option to rev-list. The output is suitable
for `eval` in shell and defines five variables:
- bisect_rev is the next revision to test.
- bisect_nr is the expected number of commits to test after
bisect_rev is tested.
- bisect_good is the expected number of commits to test
if bisect_rev turns out to be good.
- bisect_bad is the expected number of commits to test
if bisect_rev turns out to be bad.
- bisect_all is the number of commits we are bisecting right now.
The documentation text was partly stolen from Johannes
Schindelin's patch.