t5541: generalize reference locking

Some tests set up reference locks by directly creating the lockfile.
While this works for the files reference backend, reftable reference
locks operate differently and are incompatible with this approach.
Generalize reference locking by preparing a reference transaction.

Signed-off-by: Justin Tobler <jltobler@gmail.com>

t/t5541-http-push-smart.sh

@@ -232,8 +232,29 @@ test_expect_success 'push --atomic fails on server-side errors' '
 	test_config -C "$d" http.receivepack true &&
 	up="$HTTPD_URL"/smart/atomic-branches.git &&
 
-	# break ref updates for other on the remote site
-	mkdir "$d/refs/heads/other.lock" &&
+	mkfifo in out &&
+	(git -C "$d" update-ref --stdin <in >out &) &&
+
+	exec 9>in &&
+	exec 8<out &&
+	test_when_finished "exec 9>&-" &&
+	test_when_finished "exec 8<&-" &&
+
+	echo "start" >&9 &&
+	echo "start: ok" >expected &&
+	read line <&8 &&
+	echo "$line" >actual &&
+	test_cmp expected actual &&
+
+	echo "update refs/heads/other refs/heads/other" >&9 &&
+
+	# Prepare reference transaction on `other` reference to lock it and thus
+	# break updates on the remote.
+	echo "prepare" >&9 &&
+	echo "prepare: ok" >expected &&
+	read line <&8 &&
+	echo "$line" >actual &&
+	test_cmp expected actual &&
 
 	# add the new commit to other
 	git branch -f other collateral &&