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

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