diff.c: emit_add_line() takes only the rest of the line

diff.c: emit_add_line() takes only the rest of the line

As the first character on the line that is fed to this function is always
"+", it is pointless to send that along with the rest of the line.

This change will make it easier to reuse the logic when emitting the
rewrite diff, as we do not want to copy a line only to add "+"/"-"/" "
immediately before its first character when we produce rewrite diff
output.

Signed-off-by: Junio C Hamano <gitster@pobox.com>