I haven't been following the discussion around this series, but don't
we also usually provide a `-z` short option? Should that be added for
consistency with other commands and to avoid surprising users, or is
it too early to commit to that?

Yes, it is planned to be added in a future patch!

something like this:

   printf "$key\n$expected_value\0" >expect &&
   git -C repo repo info --format=nul "$key" >actual &&
   test_cmp_bin expect actual

Even better, and it's consistent to your previous suggestion. I'll
do that in the next version.