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/resmap/factory_test.go

@@ -270,7 +270,7 @@ func TestNewResMapFromSecretArgs(t *testing.T) {
 	}
 	fakeFs := fs.MakeFakeFS()
 	fakeFs.Mkdir(".")
-	rmF.Set(fakeFs, loader.NewFileLoader(fakeFs))
+	rmF.Set(fakeFs, loader.NewFileLoaderAtRoot(fakeFs))
 	actual, err := rmF.NewResMapFromSecretArgs(secrets, nil)
 
 	if err != nil {
@@ -326,7 +326,7 @@ func TestSecretTimeout(t *testing.T) {
 	}
 	fakeFs := fs.MakeFakeFS()
 	fakeFs.Mkdir(".")
-	rmF.Set(fakeFs, loader.NewFileLoader(fakeFs))
+	rmF.Set(fakeFs, loader.NewFileLoaderAtRoot(fakeFs))
 	_, err := rmF.NewResMapFromSecretArgs(secrets, nil)
 
 	if err == nil {