add tests for cloning corrupted repositories

add tests for cloning corrupted repositories

We try not to let corruption pass unnoticed over fetches and
clones. For the most part, this works, but there are some
broken corner cases, including:

  1. We do not detect missing objects over git-aware
     transports. This is a little hard to test, because the
     sending side will actually complain about the missing
     object.

     To fool it, we corrupt a repository such that we have a
     "misnamed" object: it claims to be sha1 X, but is
     really Y. This lets the sender blindly transmit it, but
     it is the receiver's responsibility to verify that what
     it got is sane (and it does not).

  2. We do not detect missing or misnamed blobs during the
     checkout phase of clone.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>