_casbaseddirectory.py: add _save() function.

buildstream/storage/_casbaseddirectory.py

@@ -561,3 +561,14 @@ class CasBasedDirectory(Directory):
         throw an exception. """
         raise VirtualDirectoryError("_get_underlying_directory was called on a CAS-backed directory," +
                                     " which has no underlying directory.")
+
+    def _save(self, name):
+        """Saves this directory into the content cache as a named ref. Used
+        by remote execution to make references for source directories so they
+        can be pushed to a remote artifact server.
+
+        """
+        self._recalculate_recursing_up()
+        self._recalculate_recursing_down()
+
+        self.cas_cache.set_ref(name, self.ref)