t5520-pull: Test for rebased upstream + fetch + pull --rebase

If your upstream has rebased you can do:

git pull --rebase

but only if you haven't fetch before.

Mark this case as test_expect_failure, in a later patch it will be
changed to test_expect_success.

Signed-off-by: Santi Béjar <santi@agolina.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>