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 can be confusing that some commits (typically the working copy)
don't have a description. Let's show a placeholder text in such cases.
I chose the format to match the "(no email configured)" message we
already have.
I originally made the operation argument a named argument
(`--operation`) to allow for a change ID to be passed as a positional
argument, matching e.g. `hg revert -r <rev> <path>`. However, even if
we add support for undoing changes only to certain change IDs, it's
going to be done much less frequently than full undo/restore. We can
therefore make that a named argument if we ever add it.
