graph: output padding for merge subsequent parents

graph: output padding for merge subsequent parents

When showing merges in git-log, the same commit is shown once for each
parent.  Combined with "--graph" this results in graph_show_commit()
being called once for each parent without graph_update() being called.

Currently graph_show_commit() does not print anything on subsequent
invocations for the same commit (this was changed by commit 656197a -
"graph.c: infinite loop in git whatchanged --graph -m" from the previous
behaviour of looping infinitely).

Change this so that if the graph code believes it has already shown the
commit it prints a single padding line.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>