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>

Author: ttaylorr

t/t5537-fetch-shallow.sh

@@ -175,7 +175,8 @@ test_expect_success 'fetch --update-shallow into a repo with submodules' '
 test_expect_success 'fetch --update-shallow a commit that is also a shallow point into a repo with submodules' '
 	test_when_finished "rm -rf repo-with-sub" &&
 	git init repo-with-sub &&
-	git -C repo-with-sub submodule add ../a-submodule a-submodule &&
+	git -c protocol.file.allow=always -C repo-with-sub \
+		submodule add ../a-submodule a-submodule &&
 	git -C repo-with-sub commit -m "added submodule" &&
 
 	SHALLOW=$(cat shallow/.git/shallow) &&