Merge branch 'jk/duplicate-objects-in-packs'

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