files-backend: add unified interface for refs scanning

files-backend: add unified interface for refs scanning

For refs and reflogs, we need to scan its corresponding directories to
check every regular file or symbolic link which shares the same pattern.
Introduce a unified interface for scanning directories for
files-backend.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>