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