Code preparation for pluggable ref backends.

* dt/refs-backend-pre-vtable:
  refs: break out ref conflict checks
  files_log_ref_write: new function
  initdb: make safe_create_dir public
  refs: split filesystem-based refs code into a new file
  refs/refs-internal.h: new header file
  refname_is_safe(): improve docstring
  pack_if_possible_fn(): use ref_type() instead of is_per_worktree_ref()
  copy_msg(): rename to copy_reflog_msg()
  verify_refname_available(): new function
  verify_refname_available(): rename function