t5516: don't use HEAD ref for invalid ref-deletion tests

A few tests in t5516 want to assert that we can delete a corrupted ref
whose pointed-to object is missing. They do so by using the "main"
branch, which is also pointed to by HEAD.

This does work, but only because of a subtle assumption about the
implementation. We do not block the deletion because of the invalid ref,
but we _also_ do not notice that the deleted branch is pointed to by
HEAD. And so the safety rule of "do not allow HEAD to be deleted in a
non-bare repository" does not kick in, and the test passes.

Let's instead use a non-HEAD branch. That still tests what we care about
here (deleting a corrupt ref), but without implicitly depending on our
failure to notice that we're deleting HEAD. That will future proof the
test against that behavior changing.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t5516-fetch-push.sh b/t/t5516-fetch-push.sh
index 4db8edd..b13553e 100755
--- a/t/t5516-fetch-push.sh
+++ b/t/t5516-fetch-push.sh
@@ -662,10 +662,10 @@
 
 test_expect_success 'allow deleting an invalid remote ref' '
 
-	mk_test testrepo heads/main &&
+	mk_test testrepo heads/branch &&
 	rm -f testrepo/.git/objects/??/* &&
-	git push testrepo :refs/heads/main &&
-	(cd testrepo && test_must_fail git rev-parse --verify refs/heads/main)
+	git push testrepo :refs/heads/branch &&
+	(cd testrepo && test_must_fail git rev-parse --verify refs/heads/branch)
 
 '
 
@@ -706,25 +706,25 @@
 '
 
 test_expect_success 'deleting dangling ref triggers hooks with correct args' '
-	mk_test_with_hooks testrepo heads/main &&
+	mk_test_with_hooks testrepo heads/branch &&
 	rm -f testrepo/.git/objects/??/* &&
-	git push testrepo :refs/heads/main &&
+	git push testrepo :refs/heads/branch &&
 	(
 		cd testrepo/.git &&
 		cat >pre-receive.expect <<-EOF &&
-		$ZERO_OID $ZERO_OID refs/heads/main
+		$ZERO_OID $ZERO_OID refs/heads/branch
 		EOF
 
 		cat >update.expect <<-EOF &&
-		refs/heads/main $ZERO_OID $ZERO_OID
+		refs/heads/branch $ZERO_OID $ZERO_OID
 		EOF
 
 		cat >post-receive.expect <<-EOF &&
-		$ZERO_OID $ZERO_OID refs/heads/main
+		$ZERO_OID $ZERO_OID refs/heads/branch
 		EOF
 
 		cat >post-update.expect <<-EOF &&
-		refs/heads/main
+		refs/heads/branch
 		EOF
 
 		test_cmp pre-receive.expect pre-receive.actual &&