Re: [PATCH 2/7] grep: use submodule-ODB-as-alternate lazy-addition

I had wondered whether we can entirely drop add_to_alternates_memory()
but I see on second reading that that's still used by
register_all_submodule_odb...(). I wonder if it can become static in
submodule.c to prevent users from dodging the envvar+BUG()?

You mean make add_to_alternates_memory() static? I'm not sure how we can
do that - alternates is a concern that extends beyond submodules.

Curious whether add_submodule_odb() can be reduced by calling
add_submodule_odb_by_path() internally.

That's a reasonable idea, but I don't think it works in this case - in
particular, add_submodule_odb_by_path() dups its argument whereas
add_submodule_odb() already has an allocated string that it is prepared
to give up ownership of. Also, add_submodule_odb_by_path() is only one
line, so it won't save us much.