t5520: use test_config to set/unset git config variables (leftover bits)

Configuration from test_config does not last beyond the end of the
current test assertion, making each test easier to think about in
isolation.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5520-pull.sh

@@ -167,9 +167,9 @@ test_expect_success 'pull --rebase dies early with dirty working directory' '
 	git update-ref refs/remotes/me/copy copy^ &&
 	COPY=$(git rev-parse --verify me/copy) &&
 	git rebase --onto $COPY copy &&
-	git config branch.to-rebase.remote me &&
-	git config branch.to-rebase.merge refs/heads/copy &&
-	git config branch.to-rebase.rebase true &&
+	test_config branch.to-rebase.remote me &&
+	test_config branch.to-rebase.merge refs/heads/copy &&
+	test_config branch.to-rebase.rebase true &&
 	echo dirty >> file &&
 	git add file &&
 	test_must_fail git pull &&