[FEAT] Don't allow modification to internal reference

- This a port of https://github.com/go-gitea/gitea/pull/31931 in a
behavior-sense. None of the code was actually ported.
- Follow up for #2834, now also don't allow modification.
- Integration test added.
- Unit test modified.

cmd/hook.go

@@ -322,7 +322,8 @@ func runHookUpdate(c *cli.Context) error {
 		return fail(ctx, fmt.Sprintf("The deletion of %s is skipped as it's an internal reference.", refFullName), "")
 	}
 
-	return nil
+	// If the new comment isn't empty it means modification.
+	return fail(ctx, fmt.Sprintf("The modification of %s is skipped as it's an internal reference.", refFullName), "")
 }
 
 func runHookPostReceive(c *cli.Context) error {

cmd/hook_test.go

@@ -180,8 +180,15 @@ func TestRunHookUpdate(t *testing.T) {
 	})
 
 	t.Run("Update of internal reference", func(t *testing.T) {
+		defer test.MockVariableValue(&cli.OsExiter, func(code int) {})()
+		defer test.MockVariableValue(&setting.IsProd, false)()
+		finish := captureOutput(t, os.Stderr)
+
 		err := app.Run([]string{"./forgejo", "update", "refs/pull/1/head", "0a51ae26bc73c47e2f754560c40904cf14ed51a9", "0000000000000000000000000000000000000001"})
-		require.NoError(t, err)
+		out := finish()
+		require.Error(t, err)
+
+		assert.Contains(t, out, "The modification of refs/pull/1/head is skipped as it's an internal reference.")
 	})
 
 	t.Run("Removal of branch", func(t *testing.T) {