submodule--helper: don't use global --super-prefix in "absorbgitdirs"

submodule--helper: don't use global --super-prefix in "absorbgitdirs"

The "--super-prefix" facility was introduced in [1] has always been a
transitory hack, which is why we've made it an error to supply it as
an option to "git" to commands that don't know about it.

That's been a good goal, as it has a global effect we haven't wanted
calls to get_super_prefix() from built-ins we didn't expect.

But it has meant that when we've had chains of different built-ins
using it all of the processes in that "chain" have needed to support
it, and worse processes that don't need it have needed to ask for
"SUPPORT_SUPER_PREFIX" because their parent process needs it.

That's how "fsmonitor--daemon" ended up with it, per [2] it's called
from (among other things) "submodule--helper absorbgitdirs", but as we
declared "submodule--helper" as "SUPPORT_SUPER_PREFIX" we needed to
declare "fsmonitor--daemon" as accepting it too, even though it
doesn't care about it.

But in the case of "absorbgitdirs" it only needed "--super-prefix" to
invoke itself recursively, and we'd never have another "in-between"
process in the chain. So we didn't need the bigger hammer of "git
--super-prefix", and the "setenv(GIT_SUPER_PREFIX_ENVIRONMENT, ...)"
that it entails.

Let's instead accept a hidden "--super-prefix" option to
"submodule--helper absorbgitdirs" itself.

Eventually (as with all other "--super-prefix" users) we'll want to
clean this code up so that this all happens in-process. I.e. needing
any variant of "--super-prefix" is itself a hack around our various
global state, and implicit reliance on "the_repository". This stepping
stone makes such an eventual change easier, as we'll need to deal with
less global state at that point.

The "fsmonitor--daemon" test adjusted here was added in [3]. To assert
that it didn't run into the "--super-prefix" message it was asserting
the output it didn't have. Let's instead assert the full output that
we *do* have, using the same pattern as a preceding change to
"t/t7412-submodule-absorbgitdirs.sh" used.

We could also remove the test entirely (as [4] did), but even though
the initial reason for having it is gone we're still getting some
marginal benefit from testing the "fsmonitor" and "submodule
absorbgitdirs" interaction, so let's keep it.

The change here to have either a NULL or non-"" string as a
"super_prefix" instead of the previous arrangement of "" or non-"" is
somewhat arbitrary. We could also decide to never have to check for
NULL.

As we'll be changing the rest of the "git --super-prefix" users to the
same pattern, leaving them all consistent makes sense. Why not pick ""
over NULL? Because that's how the "prefix" works[5], and having
"prefix" and "super_prefix" work the same way will be less
confusing. That "prefix" picked NULL instead of "" is itself
arbitrary, but as it's easy to make this small bit of our overall API
consistent, let's go with that.

1. 74866d75793 (git: make super-prefix option, 2016-10-07)
2. 53fcfbc84f6 (fsmonitor--daemon: allow --super-prefix argument,
   2022-05-26)
3. 53fcfbc84f6 (fsmonitor--daemon: allow --super-prefix argument,
   2022-05-26)
4. https://lore.kernel.org/git/20221109004708.97668-5-chooglen@google.com/
5. 9725c8dda20 (built-ins: trust the "prefix" from run_builtin(),
   2022-02-16)

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>