revision.c: implement --max-count-oldest

revision.c: implement --max-count-oldest

"--max-count" is a commit limiting option and sets a maximum amount
of commits to be shown. If a user wants to see only the first N
commits of the history (the oldest commits) they'd have to do
something like

    git log $(git rev-list HEAD | tail -n N | head -n 1)

This is not very user-friendly.

Teach get_revision() the --max-count-oldest option.

Signed-off-by: Mirko Faina <mroik@delayed.space>
[jc: fixed up t4202 <xmqq7boy4o05.fsf@gitster.g>]
Signed-off-by: Junio C Hamano <gitster@pobox.com>