Merge branch 'jc/bisect-sans-show-branch'

"git bisect" spawned "git show-branch" only to pretty-print the
title of the commit after checking out the next version to be
tested; this has been rewritten in C.

* jc/bisect-sans-show-branch:
  bisect: simplify return code from bisect_checkout()
  bisect: do not run show-branch just to show the current commit