When the git diff status 'N' was changed to 'A', diff-helper.c was
not updated accordingly.  This means that it no longer shows the
diff for newly added files.

This patch makes that change in diff-helper.c.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NJunio C Hamano <junkio@cox.net>