switch submit strategy from "cherry-pick" to "rebase always"

cf. https://codereview.qt-project.org/Documentation/config-project-config.html#submit-type

the "cherry-pick" strategy is kinda deprecated (in fact, upstream has considered removing it entirely), and "rebase always" should be used instead.

the major difference is that unlike cherry-pick, it doesn't ignore dependencies.
together with change.submitWholeTopic, this can be a real time-saver for bigger topics , esp. when the CI needs to be fought repeatedly.
on the (not really) downside, it can be a bit of a pain if one is used to sloppily pushing unrelated commits on top of each other. but this is easily manageable by standardizing on tooling which helps with this, say the gpush script.

a look at my current gerrit dashboard will reveal why i'd ask for this right now.