Merge branch 'rs/ref-transaction' into jch

Merge branch 'rs/ref-transaction' into jch

* rs/ref-transaction:
  refs.c: fix handling of badly named refs
  refs.c: make write_ref_sha1 static
  fetch.c: change s_update_ref to use a ref transaction
  refs.c: propagate any errno==ENOTDIR from _commit back to the callers
  refs.c: pass a skip list to name_conflict_fn
  refs.c: call lock_ref_sha1_basic directly from commit
  refs.c: move the check for valid refname to lock_ref_sha1_basic
  refs.c: pass NULL as *flags to read_ref_full
  refs.c: pass the ref log message to _create/delete/update instead of _commit
  refs.c: add an err argument to delete_ref_loose
  wrapper.c: add a new function unlink_or_msg
  wrapper.c: simplify warn_if_unremovable

# Conflicts:
# 	builtin/log.c
# 	git-compat-util.h
# 	refs.c
# 	walker.c