The diff information render the spec file unusable as is by p4,
do not include it when run with --prepare-p4-only so that the
given file can be directly passed to p4.

With --prepare-p4-only, git-p4 already tells the user it can use
p4 submit with the generated spec file. This fails because of the
diff being present in the file. Not including the diff fixes that.

Without --prepare-p4-only, keeping the diff makes sense for a
quick review of the patch before submitting it. And does not cause
problems with p4 as we remove it programmatically.
Signed-off-by: NMaxime Coste <frrrwww@gmail.com>
Acked-by: NPete Wyckoff <pw@padd.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>