Let's acknowledge everyone's contributions by replacing "Google LLC"
in the copyright header by "The Jujutsu Authors". If I understand
correctly, it won't have any legal effect, but maybe it still helps
reduce concerns from contributors (though I haven't heard any
concerns).
Google employees can read about Google's policy at
go/releasing/contributions#copyright.
It seems very likely that we're going to remove support for open
commits, but it's still useful to have a `commit` command that lets
the user enter a description and starts a new change. Calling it
`commit` seems good to make the transition from other VCSs simpler.
