graph.c: register a callback for graph output

graph.c: register a callback for graph output

It will look better if the 'git log --graph' print
the graph pading lines before the diff output just
like what it does for commit message.
And this patch leverage the new diff prefix callback
function to achieve this.

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