Add stack/branch parking ability

Most of the time, I have multiple stacks concurrently, and I know that a couple of them will not be merged in the near future.

It would be nice if we could mark those stacks or first branches as parked (I borrowed the term from [Git Town](https://www.git-town.com/commands/park)), so when we run a restack on the trunk (or `repo restack` #505), we would skip restacking those.

We should also add a `--force` flag to force restacking parked stacks or branches.