"git check-ref-format --branch @{-1}" bit a "BUG()" when run
outside a repository for obvious reasons; clarify the documentation
and make sure we do not even try to expand the at-mark magic in
such a case, but still call the validation logic for branch names.

* jc/check-ref-format-oor:
  check-ref-format doc: --branch validates and expands <branch>
  check-ref-format --branch: strip refs/heads/ using skip_prefix
  check-ref-format --branch: do not expand @{...} outside repository