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.
As requested by @talpr. I added this is a separate new command `jj git
remote list`. One could also imagine showing the listing when there is
no sub-command specified to `jj git remote`, but we don't have other
commands that behave that way yet.
Closes#243
