commit: avoid parent list buildup in clear_commit_marks_many()

commit: avoid parent list buildup in clear_commit_marks_many()

clear_commit_marks_1() clears the marks of the first parent and its
first parent and so on, and saves the higher numbered parents in a list
for later.  There is no benefit in keeping that list growing with each
handled commit.  Clear it after each run to reduce peak memory usage.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>