This adds the new --colocate flag to `jj git clone`.
```
jj git clone --colocate https://github.com/foo/bar
```
is effectively equivalent to:
```
git clone https://github.com/foo/bar
cd bar
jj init --git-repo=.
```
On "jj checkout", description of the working-copy commit is empty, and the
working-copy parent provides more information. It might be a bit verbose to
print parent summary on every history rewriting, but I think that's okay.
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.
Mercurial's test runner does something like this.
I considered replacing `\` by `/` everywhere, but we use `\` in
graph-log output quite frequently, so it doesn't seem worth it.
We used to check only if the destination already had a `.jj/`
directory. This patch changes that to check that the destination is an
empty directory.
Closes#399.
We didn't seem to have any. I included a test for #399.
The tests also showed how the debug formatting (`{:?}`) results in
escaped (e.g.) backslashes on Windows, which is not what we want, so I
also fixed that.
