diff --stat: do not do its own three-dashes.

I missed that "git-diff-* --stat" spits out three-dash separator
on its own without being asked.  Remove it.

When we output commit log followed by diff, perhaps --patch-with-stat,
for downstream consumer, we _would_ want the three-dash between
the message and the diff material, but that logic belongs to the
caller, not diff generator.

Signed-off-by: Junio C Hamano <junkio@cox.net>