lock_ref_sha1_basic does not remove empty directories on BSD

lock_ref_sha1_basic does not remove empty directories on BSD

lock_ref_sha1_basic relies on errno beeing set to EISDIR by the
call to read() in resolve_ref() to detect directories.  But calling
read() on a directory under NetBSD returns EPERM, and even succeeds
for local filesystems on FreeBSD.

Signed-off-by: Dennis Stosberg <dennis@stosberg.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>