Consult history in fileloader.

Fixes #366

To reproduce #366, add

```
bases:
- .
```

to `examples/helloWorld/kustomization.yaml`, attempt to build it, and enjoy the stack overflow.

This PR fixes this by adding history to file loaders,
allowing one to avoid cycles in overlay->base
relationships.  To make entry points clearer, this PR
exposes only two public ways to make a fresh
(no-history) loader

 * rooted at `/`
 * rooted at the process's current working directory.

When making a new loader from an existing loader,
retaining history along an overlay trace, the only
allowed use is to go deeper into a file hierarchy, or
go up and over to a never before visited sibling. This
fix can probably be defeated by devious symbolic links.

pkg/loader/githubloader.go

@@ -71,7 +71,7 @@ func newGithubLoader(repoUrl string, fs fs.FileSystem) (*githubLoader, error) {
 		return nil, err
 	}
 	target := filepath.Join(repodir, subdir)
-	l := newFileLoaderAtRoot(target, fs)
+	l, _ := newFileLoaderAt(fs, target)
 	return &githubLoader{
 		repo:        repoUrl,
 		targetDir:   target,