Merge branch 'rs/ref-transaction-1' into jch

Merge branch 'rs/ref-transaction-1' into jch

The second batch of the transactional ref update series.

No more nitpicks and better design suggestions?

* rs/ref-transaction-1:
  refs.c: make delete_ref use a transaction
  refs.c: make prune_ref use a transaction to delete the ref
  refs.c: remove lock_ref_sha1
  refs.c: remove the update_ref_write function
  refs.c: remove the update_ref_lock function
  refs.c: make lock_ref_sha1 static
  walker.c: use ref transaction for ref updates
  fast-import.c: use a ref transaction when dumping tags
  receive-pack.c: use a reference transaction for updating the refs
  refs.c: change update_ref to use a transaction
  branch.c: use ref transaction for all ref updates
  fast-import.c: change update_branch to use ref transactions
  sequencer.c: use ref transactions for all ref updates
  commit.c: use ref transactions for updates
  replace.c: use the ref transaction functions for updates
  tag.c: use ref transactions when doing updates
  refs.c: add transaction.status and track OPEN/CLOSED/ERROR
  refs.c: make ref_transaction_begin take an err argument
  refs.c: update ref_transaction_delete to check for error and return status
  refs.c: change ref_transaction_create to do error checking and return status

# Conflicts:
# 	sequencer.c
# 	walker.c