This allows us to use "if(description,)" to test empty description. And
I think this change is unavoidable if we want to add support for commit
template.
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I originally made the operation argument a named argument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.
