Merge branch 'jh/clone-packed-refs'

* jh/clone-packed-refs:
  Teach "git clone" to pack refs
  Prepare testsuite for a "git clone" that packs refs
  Move pack_refs() and friends into libgit
  Incorporate fetched packs in future object traversal

diff --cc Makefile
Simple merge

diff --cc builtin-clone.c
Simple merge