data tree CHANGE switch diff merge parameters

SO that it follows general merge prototype.
diff --git a/src/validation.c b/src/validation.c
index 6760c13..2d71846 100644
--- a/src/validation.c
+++ b/src/validation.c
@@ -98,7 +98,7 @@
     LY_CHECK_RET(lyd_diff_add(node, op, NULL, NULL, NULL, NULL, NULL, &new_diff));
 
     /* merge into existing diff */
-    ret = lyd_diff_merge_all(new_diff, diff);
+    ret = lyd_diff_merge_all(diff, new_diff);
 
     lyd_free_tree(new_diff);
     return ret;