Arthur: update tf-sync-repos.yaml

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I526f5b8fc9a279f200ac6f3e4cbb17b3c23d35cd
diff --git a/tf-sync-repos.yaml b/tf-sync-repos.yaml
index 448da94..fd7ccc6 100644
--- a/tf-sync-repos.yaml
+++ b/tf-sync-repos.yaml
@@ -26,6 +26,7 @@
     scm:
         - git:
             url: https://review.trustedfirmware.org/${GERRIT_PROJECT}
+            refspec: ${GERRIT_REFSPEC}
             branches:
                 - origin/master
             skip-tag: false