t3207: prepare for changing protocol.file.allow

Explicitly cloning over the "file://" protocol in t3207 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/t3207-branch-submodule.sh

@@ -28,6 +28,7 @@ test_no_branch () {
 }
 
 test_expect_success 'setup superproject and submodule' '
+	git config --global protocol.file.allow always &&
 	mkdir test_dirs &&
 	(
 		cd test_dirs &&