ref-filter: add test for --contains on a non-commit

ref-filter: add test for --contains on a non-commit

Change the tag test suite to test for --contains on a tree & blob. It
only accepts commits and will spew out "<object> is a tree, not a
commit".

It's sufficient to test this just for the "tag" and "branch" commands,
because it covers all the machinery shared between "branch" and
"for-each-ref".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>