The simplify_merges() function needs to look at all history chain to
find the closest ancestor that is relevant after the simplification,
but after --first-parent traversal, side parents haven't been marked
for relevance (they are irrelevant by definition due to the nature
of first-parent-only traversal) nor culled from the parents list of
resulting commits.

We cannot simply remove these side parents from the parents list, as
the output phase still wants to see the parents.  Instead, teach
simplify_one() and its callees to ignore the later parents.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>