Branch both RC and Release from the "cut" commit

[TombWater]

We currently branch Release from the commit before the "cut" commit, and RC is branched from the "cut" commit. Because the base of these two branches differ, it's not possible to fast-forward merge RC to Release at the end of the testing cycle, and we end up with a merge commit having two parents. Those two parents make a more complicated history where it may be unclear where the release actually came from because the left parent will be the Main branch so the release version tag could be mistaken to be referring to the change before the "cut" commit.

We should change the automation to cut both the Release and RC branches from the "cut" commit.

We would then have the problem that if no fixes are done on RC then it wouldn't be possible to have a PR from RC to Release to run automations for the actual release when testing is done. That can be solved by having the cut-automation push an empty commit to RC immediately after creating it so that there's always guaranteed to be something on RC that can be merged to Release at the end.