t5537: prepare for changing protocol.file.allow

Explicitly cloning over the "file://" protocol in t5537 in preparation
for merging a security release which will change the default value of
this configuration to be "user".

Signed-off-by: Taylor Blau <me@ttaylorr.com>

t/t5537-fetch-shallow.sh

@@ -162,6 +162,8 @@ test_expect_success 'fetch --update-shallow' '
 '
 
 test_expect_success 'fetch --update-shallow into a repo with submodules' '
+	test_config_global protocol.file.allow always &&
+
 	git init a-submodule &&
 	test_commit -C a-submodule foo &&
 	git init repo-with-sub &&