A packfile that stores the same object more than once is broken and
will be rejected by "git index-pack" that is run when receiving data
over the wire.
* jk/duplicate-objects-in-packs:
t5308: check that index-pack --strict detects duplicate objects
test index-pack on packs with recoverable delta cycles
add tests for indexing packs with delta cycles
sha1-lookup: handle duplicate keys with GIT_USE_LOOKUP
test-sha1: add a binary output mode