Currently `RepoTree.open` will throw FileNotFoundError if you have a
file that is tracked by dvc, but which has no md5 or a cache for which
is missing, even though you have the file itself in your workspace.
This corresponds very nicely to the `--no-commit` or `--no-exec`
scenario, where you might not want to commit your artifacts to cache
just yet.
Fixes#3974