Currently, the documentation suggests that 'git merge-base -a' and 'git
show-branch --merge-base' are equivalent (in fact it claims that the
former cannot handle more than two revs).
Alas, the handling of more than two revs is very different. Document
this by tests and correct the documentation to reflect this.
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>