Load original_content from disk if needed to calculate updated

src/ansiblelint/file_utils.py

@@ -216,7 +216,10 @@ def content(self, value: str) -> None:
         if self._original_content is None:
             if self._content is not None:
                 self._original_content = self._content
-            elif not self.path.exists():
+            elif self.path.exists():
+                # use self.content to populate self._original_content
+                _ = self.content
+            else:
                 # new file
                 self._original_content = ""
         self.updated = self._original_content != value