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/transformers/config/factory_test.go

@@ -39,7 +39,7 @@ namePrefix:
 `
 	fakeFS := fs.MakeFakeFS()
 	fakeFS.WriteFile("/transformerconfig/test/config.yaml", []byte(transformerConfig))
-	ldr := loader.NewFileLoader(fakeFS)
+	ldr := loader.NewFileLoaderAtRoot(fakeFS)
 	expected := &TransformerConfig{
 		NamePrefix: []FieldSpec{
 			{