t1507: Test that branchname@{upstream} is interpreted as branch

Syntax branchname@{upstream} should interpret its argument as a name of
a branch. Add the test to check that it doesn't try to interpret it as a
refname if the branch in question does not exist.

Signed-off-by: Kacper Kornet <draenog@pld-linux.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>