diff: add diff_line_prefix function

diff: add diff_line_prefix function

This is a helper function to call the diff output_prefix function and
return its value as a C string, allowing us to greatly simplify
everywhere that needs to get the output prefix.

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