t1400: test one mistake at a time

t1400: test one mistake at a time

This case wants to test passing a bad refname to the "update" command.
But it also passes too few arguments to "update", which muddles the
situation: which error should be diagnosed?  So split this test into
two:

* One that passes too few arguments to update

* One that passes all three arguments to "update", but with a bad
  refname.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>