merge-base --independent to print reduced parent list in a merge
While show-branch --independent does not support more than MAX_REVS
revs, git internally supports more with a different algorithm.
Expose that functionality as "git merge-base --independent".
This should help scripts to catch up with builtin merge in supporting
dodecapus.
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>