blame: allow blame --reverse --first-parent when it makes sense

blame: allow blame --reverse --first-parent when it makes sense

Allow combining --reverse and --first-parent if initial commit of
specified range is at the first-parent chain starting from the final
commit. Disable the prepare_revision_walk()'s builtin children
collection, instead picking only the ones which are along the first
parent chain.

Signed-off-by: Max Kirillov <max@max630.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>