fast-export: ensure we traverse commits in topological order

fast-export: ensure we traverse commits in topological order

fast-export will only list as parents those commits which have already
been traversed (making it appear as if merges have been squashed if not
all parents have been traversed).  To avoid this silent squashing of
merge commits, we request commits in topological order.

Signed-off-by: Elijah Newren <redacted>
Acked-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>