patch, 3 partsAvoid forking a process when checking merge bases

The goal of this patch series is to check if good revisions are ancestor of the bad revision without forking a process to launch "git rev-list $good ^$bad".

To do that it introduces an "unparse_commit" function that may be usefull in other cases, for example see:

http://article.gmane.org/gmane.comp.version-control.git/114916

Ps: sorry to send this cover letter late because of a bug in the
script I used to send the series.

  bisect: rework some rev related functions to make them more reusable
  commit: add function to unparse a commit and its parents
  bisect: check ancestors without forking a "git rev-list" process

 bisect.c                    |   79 
+++++++++++++++++++-----------------------
 commit.c                    |   20 +++++++++++
 commit.h                    |    2 +
 t/t6030-bisect-porcelain.sh |   13 +------
 4 files changed, 59 insertions(+), 55 deletions(-)