Rollback if nothing changed (#887)

Since the module unconditionally issues ALTER statements in order to
observe their effect on the postgres catalog - to determine whether the
privileges have changes - a rollback is thus advisable when in fact
nothing has changed.

fix #885

plugins/modules/database/postgresql/postgresql_privs.py

@@ -1102,7 +1102,7 @@ def main():
         conn.rollback()
         module.fail_json(msg=to_native(e.message))
 
-    if module.check_mode:
+    if module.check_mode or not changed:
         conn.rollback()
     else:
         conn.commit()