In many scripted Porcelain commands, we find this idiom:
if test "$(git rev-parse --verify A)" = "$(git merge-base A B)"
then
... A is an ancestor of B ...
fi
But you do not have to compute exact merge-base only to see if A is
an ancestor of B. Give them a more direct way to use the underlying
machinery.
Signed-off-by: Junio C Hamano <gitster@pobox.com>