get_name_branch_path_and_refname() resolved the committish with
committish_to_refname(), which dereferences the ref's name. A bare SHA
resolves to a commit with no ref, so that raised AttributeError
(NoneType has no attribute name). Fall back to the committish itself when
it has no ref, so `worktree add <path> <sha>` can branch at a specific
commit.