Merge branch 'jc/check-ref-format-oor'

"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