commit-tree: lift completely arbitrary limit of 16 parents

commit-tree: lift completely arbitrary limit of 16 parents

There is no really good reason to have a merge with more than 16
parents, but we have a history of giving our users rope.

Combined with the fact that there was no good reason for that
arbitrary limit in the first place, here is an all-too-easy to fix.

Kind of wished-for by Len Brown.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>