revision.c: don't show all merges for --parents

revision.c: don't show all merges for --parents

When using --parents or --children, get_commit_action() previously showed
all merges, even if TREESAME to both parents.

This was intended to tie together the topology of the rewritten parents,
but it was excessive - in fact we only need to show merges that have two
or more relevant parents. Merges at the boundary do not necessarily need
to be shown.

Signed-off-by: Kevin Bracey <kevin@bracey.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>