builtin/diagnose.c: create 'git diagnose' builtin

builtin/diagnose.c: create 'git diagnose' builtin

Create a 'git diagnose' builtin to generate a standalone zip archive of
repository diagnostics.

The "diagnose" functionality was originally implemented for Scalar in
aa5c79a331 (scalar: implement `scalar diagnose`, 2022-05-28). However, the
diagnostics gathered are not specific to Scalar-cloned repositories and
can be useful when diagnosing issues in any Git repository.

Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Helped-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>