t5541: remove lockfile creation

To create error conditions, some tests set up reference locks by
directly creating its lockfile. While this works for the files reference
backend, this approach is incompatible with the reftable backend.
Refactor the test to create a d/f conflict via git-update-ref(1) instead
so that the test can be reference backend agnostic.

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

Author: jltobler

t/t5541-http-push-smart.sh

@@ -232,27 +232,19 @@ 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 "$d/refs/heads/other.lock" &&
+	# Create d/f conflict to break ref updates for other on the remote site.
+	git -C "$d" update-ref -d refs/heads/other &&
+	git -C "$d" update-ref refs/heads/other/conflict HEAD &&
 
 	# add the new commit to other
 	git branch -f other collateral &&
 
 	# --atomic should cause entire push to be rejected
 	test_must_fail git push --atomic "$up" atomic other 2>output  &&
 
-	# the new branch should not have been created upstream
-	test_must_fail git -C "$d" show-ref --verify refs/heads/atomic &&
-
-	# upstream should still reflect atomic2, the last thing we pushed
-	# successfully
-	git rev-parse atomic2 >expected &&
-	# ...to other.
-	git -C "$d" rev-parse refs/heads/other >actual &&
-	test_cmp expected actual &&
-
-	# the new branch should not have been created upstream
+	# The atomic and other branches should be created upstream.
 	test_must_fail git -C "$d" show-ref --verify refs/heads/atomic &&
+	test_must_fail git -C "$d" show-ref --verify refs/heads/other &&
 
 	# the failed refs should be indicated to the user
 	grep "^ ! .*rejected.* other -> other .*atomic transaction failed" output &&