Merge branch 'jk/peel-ref'

Merge branch 'jk/peel-ref'

Speeds up "git upload-pack" (what is invoked by "git fetch" on the
other side of the connection) by reducing the cost to advertise the
branches and tags that are available in the repository.

* jk/peel-ref:
  upload-pack: use peel_ref for ref advertisements
  peel_ref: check object type before loading
  peel_ref: do not return a null sha1
  peel_ref: use faster deref_tag_noverify