Files already exist in <location> when migrating a repo.

Ok, back again :)

I was able to get copybara working using the copybara repo, and now I'm switching over to the @sonatype repo I want to migrate to a public one.

I'm running `copybara copy.bara.sky --force`

My copy.bara.sky looks like:

```
url = "git@github.com:sonatype/INTERNAL_REPO.git"

core.workflow(
name = "default",
origin = git.origin(
url = url,
ref = "master",
),
destination = git.destination(
url = "file:///tmp/foo",
fetch = "master",
push = "master",
),
destination_files = glob(["matthew/py-iq-reports/IQreport/**"], exclude = ["README_INTERNAL.txt"]),

authoring = authoring.pass_thru("Jeffry Hesse "),
transformations = [
core.move("", ""),
],
)
```

I end up with this:

```
764 jeffryhesse:IQreport (master *)$ copybara copy.bara.sky --force
Copybara source mover (Version: Unknown version)
Task: Git Origin: Initializing local repo
Enter passphrase for key '/Users/jeffryhesse/.ssh/id_rsa':
WARN: Cannot find last imported revision, but proceeding because of --force flag
Task: Moving
ERROR: Files already exist in /Users/jeffryhesse/copybara/temp/workdir2564596437170999916/checkout:
```

I've truncated the filelist since it's a lot of internal stuff, plus likely not going to give much actual detail on the issue. I'm not sure what's quite going on here, and I'm sure I'm missing something. Any help is appreciated.