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.

k8sdeps/configmapandsecret/configmapfactory_test.go

@@ -131,7 +131,7 @@ func TestConstructConfigMap(t *testing.T) {
 	fSys := fs.MakeFakeFS()
 	fSys.WriteFile("/configmap/app.env", []byte("DB_USERNAME=admin\nDB_PASSWORD=somepw\n"))
 	fSys.WriteFile("/configmap/app-init.ini", []byte("FOO=bar\nBAR=baz\n"))
-	f := NewConfigMapFactory(fSys, loader.NewFileLoader(fSys))
+	f := NewConfigMapFactory(fSys, loader.NewFileLoaderAtRoot(fSys))
 	for _, tc := range testCases {
 		cm, err := f.MakeConfigMap(&tc.input, tc.options)
 		if err != nil {