You can do both. In all fairness, we do not enforce a particular working style with Wireshark, but please keep in mind, that other people must review the stuff. That being said, it usually goes with - it should compile and work - after commit. So submitting things, where you and up with a broken mainline will not fly.
And for gerri, gerri let's you start special branches as well, they are called "topic". Branches exist as well, but usually if you have more than one patchset, which depend on each other, you give all of them the same topic.
cheers
Roland