line-log: get rid of the parents array in process_ranges_merge_commit()

line-log: get rid of the parents array in process_ranges_merge_commit()

We can easily iterate through the parents of a merge commit without
turning the list of parents into a dynamically allocated array of
parents, so let's do so.  This way we can avoid a memory allocation
for each processed merge commit, though its effect on runtime seems to
be unmeasurable.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>