Merge branch 'bc/clone-empty-repo-via-protocol-v0'

The server side of "git clone" now advertises the necessary hints
to clients to help them to clone from an empty repository and learn
object hash algorithm and the (unborn) branch pointed at by HEAD,
even over the older v0/v1 protocol.

* bc/clone-empty-repo-via-protocol-v0:
  upload-pack: advertise capabilities when cloning empty repos
