hugolib: Fix IsTranslated for "old" node types

The new logic for creating Page objects from old node types
didn't include itself in the translation logic, so
`IsTranslated` returned falsely false for sites with only two languages.

The `AllTranslations` method also returned too few pages in that case.

This commit fixes that.

Fixes #2812

hugolib/hugo_sites.go

@@ -180,7 +180,7 @@ func (h *HugoSites) assignMissingTranslations() error {
 		// Assign translations
 		for _, t1 := range nodes {
 			for _, t2 := range nodes {
-				if t2.isNewTranslation(t1) {
+				if t1.isNewTranslation(t2) {
 					t1.translations = append(t1.translations, t2)
 				}
 			}