t0411: add tests for cloning from partial repo

t0411: add tests for cloning from partial repo

Cloning from a partial repository must not fetch missing objects into
the partial repository, because that can lead to arbitrary code
execution.

Add a couple of test cases, pretending to the `upload-pack` command (and
to that command only) that it is working on a repository owned by
someone else.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Filip Hejsek <filip.hejsek@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>