Merge branch 'ma/create-pseudoref-with-null-old-oid'

"git update-ref A B" is supposed to ensure that ref A does not yet exist when B is a NULL OID, but this check was not done correctly for pseudo-refs outside refs/ hierarchy, e.g. MERGE_HEAD. * ma/create-pseudoref-with-null-old-oid: refs: handle zero oid for pseudorefs t1400: add tests around adding/deleting pseudorefs refs.c: refer to "object ID", not "sha1", in error messages