t3206: prepare for changing protocol.file.allow

Explicitly cloning over the "file://" protocol in t3206 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/t3206-range-diff.sh

@@ -782,7 +782,7 @@ test_expect_success 'submodule changes are shown irrespective of diff.submodule'
 	sub_oid3=$(git -C sub-repo rev-parse HEAD) &&
 
 	git checkout -b main-sub topic &&
-	git submodule add ./sub-repo sub &&
+	git -c protocol.file.allow=always submodule add ./sub-repo sub &&
 	git -C sub checkout --detach sub-first &&
 	git commit -m "add sub" sub &&
 	sup_oid1=$(git rev-parse --short HEAD) &&