grep: use submodule-ODB-as-alternate lazy-addition

grep: use submodule-ODB-as-alternate lazy-addition

In the parent commit, Git was taught to add submodule ODBs as alternates
lazily, but grep does not use this because it computes the path to add
directly, not going through add_submodule_odb(). Add an equivalent to
add_submodule_odb() that takes the exact ODB path and teach grep to use
it.

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>