Merge branch 'jc/push-cert' into pu

Merge branch 'jc/push-cert' into pu

Allow "git push" request to be signed, so that it can be verified
audited, using the GPG signature of the person who pushed, that the
tips of branches at a public repository really point the commits
the pusher wanted to, without having to "trust" the server.

Replay attack prevention needs to be tightened and also need to fix
a few minor typos.

* jc/push-cert:
  signed-push: add "pushee" header to push certificate
  signed push: fortify against replay attacks
  signed push: remove duplicated protocol info
  send-pack: send feature request on push-cert packet
  receive-pack: GPG-validate push certificates
  the beginning of the signed push
  pack-protocol doc: typofix for PKT-LINE
  gpg-interface: move parse_signature() to where it should be
  gpg-interface: move parse_gpg_output() to where it should be
  send-pack: clarify that cmds_sent is a boolean
  send-pack: refactor inspecting and resetting status and sending commands
  send-pack: rename "new_refs" to "need_pack_data"
  send-pack: factor out capability string generation
  send-pack: always send capabilities
  send-pack: refactor decision to send update per ref
  send-pack: move REF_STATUS_REJECT_NODELETE logic a bit higher
  receive-pack: factor out queueing of command
  receive-pack: do not reuse old_sha1[] for other things
  receive-pack: parse feature request a bit earlier
  receive-pack: do not overallocate command structure

# Conflicts:
# 	builtin/receive-pack.c