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.
