t5540: require REFFILES

The dumb HTTP protocol exposes ref storage details as part of the protocol,
so it only works with the FILES refstorage backend

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5540-http-push-webdav.sh

@@ -18,6 +18,12 @@ then
 	test_done
 fi
 
+if test_have_prereq !REFFILES
+then
+	skip_all='skipping test; dumb HTTP protocol not supported with reftable.'
+	test_done
+fi
+
 LIB_HTTPD_DAV=t
 . "$TEST_DIRECTORY"/lib-httpd.sh
 ROOT_PATH="$PWD"