dim: set magic fdo cookie push-option on git push

Make it possible for the server side git pre-receive hook to determine
the client is dim and refuse the push otherwise.

Obviously it's possible to set the push-option manually on the git
command line as well, but there's really no plausible denial in that
case.

Cc: Daniel Stone <daniel@fooishbar.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>