Interesting (I'm not a kernel developer). What's the workflow at the subsystem level -- does the maintainer do a `git fetch` followed by `git rebase`? Or is the rebasing done via email patches and `git am`?
Maintainers all have their own tools and scripts to help automate and track the whole process - in the area where I work, we track things using Patchwork (see http://patchwork.ozlabs.org/project/linuxppc-dev/list/). When the maintainer's happy with a patch, it gets applied and pushed to one of the trees they use (e.g. with powerpc, we have powerpc-next for feature development and powerpc-fixes for important fixes). Eventually, they send Linus a pull request, and it makes its way into the kernel mainline.
It's a bit of a tricky system that requires understanding of the kernel community's social norms to get right - which I'm not entirely happy with but I don't think it'll change particularly quickly. However, it's also surprisingly effective - the kernel is one of the largest and most distributed individual projects in the open source world, and as a community we keep pushing out releases.