diff.c: Output the text graph padding before each diff line

Change output from diff with -p/--dirstat/--binary/--numstat/--stat/
--shortstat/--check/--summary options to align with graph paddings.

Thanks Jeff King <peff@peff.net> for reporting the '--summary' bug and
his initial patch.

Signed-off-by: Bo Yang <struggleyb.nku@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>