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>