git-rev-list: add --bisect-vars option.

git-rev-list: add --bisect-vars option.

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.

Signed-off-by: Junio C Hamano <junkio@cox.net>