Add test for cloning with "--reference" repo being a subset of source repo

Add test for cloning with "--reference" repo being a subset of source repo

The first test in this series tests "git clone -l -s --reference B A C",
where repo B is a superset of repo A (A has one commit, B has the same
commit plus another). In this case, all objects to be cloned are already
present in B.

However, we should also test the case where the "--reference" repo is a
_subset_ of the source repo (e.g. "git clone -l -s --reference A B C"),
i.e. some objects are not available in the "--reference" repo, and will
have to be found in the source repo.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>