attr: read attributes from HEAD when bare repo

attr: read attributes from HEAD when bare repo

The motivation for 44451a2e5e (attr: teach "--attr-source=<tree>" global
option to "git" , 2023-05-06), was to make it possible to use
gitattributes with bare repositories.

To make it easier to read gitattributes in bare repositories however,
let's just make HEAD:.gitattributes the default. This is in line with
how mailmap works, 8c473cecfd (mailmap: default mailmap.blob in bare
repositories, 2012-12-13).

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>