refs.c allow multiple updates of the same ref in a transaction

Allow multiple updates of a ref in the same transaction as long as
each update has have_old and old_sha1 matches the new_sha1 of the
previous update.

Add a test that verifies that a valid sequence such as
  create ref a
  update ref b a
  update ref c b
works and a test that an invalid sequence such as this still fails:
  update ref a c
  update ref b b
  update ref c c

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>