format-patch: --numbered-files and --stdout aren't mutually exclusive

For example:

git format-patch --numbered-files --stdout --attach HEAD~~

will create two messages with files 1 and 2 attached respectively.
There is no effect when using --numbered-files and --stdout together
without an --attach or --inline, the --numbered-files option will be
ignored. Add a test to show this.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>