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 `next invocation` command to `fake-editor` that ignores the
rest of the script for *this* invocation, but overwrites the script
with whatever follows `next invocation`.
This is useful to test commands that invoke `fake-editor` several times,
especially the `jj split` command.
