Add test for empty push

Add a test to ensure pushing a repo that doesn't have any additional
changes doesn't leave stale metadata behind.

test/push-after-push-no-changes.t

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+set -e
+
+source test/setup
+
+use Test::More
+
+clone-foo-and-bar
+subrepo-clone-bar-into-foo
+
+{
+  message=$(
+    cd "$OWNER/foo"
+    git subrepo push bar > /dev/null
+    add-new-files bar/Bar1
+    catch git subrepo push bar
+  )
+
+  is "$message" \
+    "Subrepo 'bar' pushed to '$UPSTREAM/bar' (master)." \
+    "Output OK: Check that 'push' after an empty push works."
+}
+
+done_testing 1
+
+teardown