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.
This adds a warning whenever export to the backing Git repo fails,
whether it's by an explicit `jj git export` or an automatic export. It
might be too spammy to print the message after every failed command in
the colocated case, but let's try it and see.
