t7814: show lack of alternate ODB-adding

t7814: show lack of alternate ODB-adding

The previous patches have made "git grep" no longer need to add
submodule ODBs as alternates, at least for the code paths tested in
t7814. Demonstrate this by making adding a submodule ODB as an alternate
fatal in this test.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Reviewed-by: Emily Shaffer <emilyshaffer@google.com>
Reviewed-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>