Making errno when returning from resolve_ref_unsafe() meaningful,
which should fix

 * a bug in lock_ref_sha1_basic, where it assumes EISDIR
   means it failed due to a directory being in the way
Signed-off-by: NRonnie Sahlberg <sahlberg@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>
Acked-by: NMichael Haggerty <mhagger@alum.mit.edu>