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>