revup sync

[jerry-skydio]

Create a tool combining the following functionalities:

Ability to do the equivalent of "git rebase --onto" a relative branch, which is useful if working relative to a branch with force-pushes / rewritten history (such as other revup branches)

Ability to do a "git pull --rebase" in-place, so that user doesn't get build churn by dropping and then reapplying local changes

[jerry-skydio]

duplicated by #90