patman: Add --no-signoff to suppress adding signoffs

To enable use of patman with FSF/GNU projects, such as GCC or
Binutils, no Signed-off-by may be added.  This adds a command
line flag '--no-signoff' to suppress adding signoffs in patman
when processing commits.

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reviewed-by: Simon Glass <sjg@chromium.org>
Fix patman testBranch() test:
Signed-off-by: Simon Glass <sjg@chromium.org>