Fix missing parameter in promote

Change-Id: Iff32b854155bd84bb0c2c8b422f2dd5de3350fa0
diff --git a/src/com/mirantis/mk/Aptly.groovy b/src/com/mirantis/mk/Aptly.groovy
index fdf0d27..4979ac4 100644
--- a/src/com/mirantis/mk/Aptly.groovy
+++ b/src/com/mirantis/mk/Aptly.groovy
@@ -85,7 +85,7 @@
         opts = "${opts} --recreate"
     }
     if (diff.toBoolean() == true) {
-        opts = "--dry --diff"
+        opts = "${opts} --dry --diff"
     }
 
     sh("aptly-publisher --url ${server} promote --source ${source} --target ${target} ${opts}")