tests: at-combinations: simplify setup

The test is setting up an upstream branch, but there's a much simpler
way of doing that: git branch -u.

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

Author: Felipe Contreras

t/t1508-at-combinations.sh

@@ -31,10 +31,8 @@ test_expect_success 'setup' '
 	git checkout -b new-branch &&
 	test_commit new-one &&
 	test_commit new-two &&
-	git config branch.old-branch.remote . &&
-	git config branch.old-branch.merge refs/heads/master &&
-	git config branch.new-branch.remote . &&
-	git config branch.new-branch.merge refs/heads/upstream-branch
+	git branch -u master old-branch &&
+	git branch -u upstream-branch new-branch
 '
 
 check HEAD new-two