PlainClone breaking change for returned error in v4.8

[jstangroome]

In v4.7.1, git.PlainClone(path, ...) would error with git.ErrRepositoryAlreadyExists if the path directory was already populated with a git repository.

In v4.8.0 the same call now returns an arbitrary fmt.Errorf("directory is not empty: %s", path) and breaks code which was checking for err == git.ErrRepositoryAlreadyExists.

Responsible code change appears to be: 3332e8d

[smola]

@jstangroome Thanks for reporting. I'm taking care of this ASAP.

[smola]

Fix proposed in https://github.com/src-d/go-git/pull/1028/files
For go-git v5, I propose to change this behavior and refuse upfront to clone on a non-empty dir, without even contacting remote server. Checking if a remote exists without creating a repository should be accomplished by a git remote-ls-like API.