patman: Add an option to create patches without binary contents

patman: Add an option to create patches without binary contents

Some mailing lists have size limits and when we add binary contents
to our patches it's easy to exceed the size limits.

Git supports a command line option "--no-binary" to generate patches
without any binary contents. Add an option in patman to handle this.
Note with this option patches cannot be applied properly, but they
are still useful for code review.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>