Merge branch 'lt/refs' into jc/lt-ref2-with-lt-refs

Merge branch 'lt/refs' into jc/lt-ref2-with-lt-refs

* lt/refs: (58 commits)
  git-pack-refs --prune
  pack-refs: do not pack symbolic refs.
  Tell between packed, unpacked and symbolic refs.
  Add callback data to for_each_ref() family.
  symbolit-ref: fix resolve_ref conversion.
  Fix broken sha1 locking
  fsck-objects: adjust to resolve_ref() clean-up.
  gitignore: git-pack-refs is a generated file.
  wt-status: use simplified resolve_ref to find current branch
  Fix t1400-update-ref test minimally
  Enable the packed refs file format
  Make ref resolution saner
  Add support for negative refs
  Start handling references internally as a sorted in-memory list
  gitweb fix validating pg (page) parameter
  git-repack(1): document --window and --depth
  git-apply(1): document --unidiff-zero
  gitweb: fix warnings in PATH_INFO code and add export_ok/strict_export
  upload-archive: monitor child communication even more carefully.
  gitweb: export options
  ...