Rollback if nothing changed (#887) (#925)

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

(cherry picked from commit 2b3c8f4582)

Co-authored-by: Georg Sauthoff <mail@georg.so>

changelogs/fragments/887-rollback-if-nothing-changed.yml

@@ -0,0 +1,2 @@
+bugfixes:
+- postgresql_privs - rollback if nothing changed (https://github.com/ansible-collections/community.general/issues/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()