cli: Add "--signoff" option to apply subcommand
cli: Add "--signoff" option to apply subcommand

If specified, an extra "signed-off-by" line will be added to the commit
once applied.

Signed-off-by: Fam Zheng <df9fd617995bad07eac70e27aa3fe5069270e3bd@redhat.com>

famz/patchew,patchew-project/patchew,famz/patchew,patchew-project/patchew,famz/patchew,famz/patchew,patchew-project/patchew,patchew-project/patchew