revision.c: use commit-slab for show_source

revision.c: use commit-slab for show_source

Instead of relying on commit->util to store the source string, let the
user provide a commit-slab to store the source strings in.

It's done so that commit->util can be removed. See more explanation in
the commit that removes commit->util.

Signed-off-by: Junio C Hamano <gitster@pobox.com>