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

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

Cover updates to reflog with the same transaction mechanism as used
for reflog manipulations.

No more nitpicks and better design suggestions?

* rs/ref-transaction-reflog:
  refs.c: allow deleting refs with a broken sha1
  refs.c: remove lock_any_ref_for_update
  refs.c: make unlock_ref/close_ref/commit_ref static
  refs.c: rename log_ref_setup to create_reflog
  reflog.c: use a reflog transaction when writing during expire
  refs.c: allow multiple reflog updates during a single transaction
  refs.c: only write reflog update if msg is non-NULL
  refs.c: add a flag to allow reflog updates to truncate the log
  refs.c: add a transaction function to append a reflog entry
  lockfile.c: make hold_lock_file_for_append preserve meaningful errno
  refs.c: add a function to append a reflog entry to a fd
  refs.c: add a new update_type field to ref_update
  refs.c: rename the transaction functions
  refs.c: make ref_transaction_delete a wrapper for ref_transaction_update
  refs.c: make ref_transaction_create a wrapper to ref_transaction_update

# Conflicts:
# 	builtin/checkout.c
# 	builtin/reflog.c
# 	refs.c
# 	refs.h
# 	walker.c