builtin/bugreport.c: create '--diagnose' option

builtin/bugreport.c: create '--diagnose' option

Create a '--diagnose' option for 'git bugreport' to collect additional
information about the repository and write it to a zipped archive.

The '--diagnose' option behaves effectively as an alias for simultaneously
running 'git bugreport' and 'git diagnose'. In the documentation, users are
explicitly recommended to attach the diagnostics alongside a bug report to
provide additional context to readers, ideally reducing some back-and-forth
between reporters and those debugging the issue.

Note that '--diagnose' may take an optional string arg (either 'stats' or
'all'). If specified without the arg, the behavior corresponds to running
'git diagnose' without '--mode'. As with 'git diagnose', this default is
intended to help reduce unintentional leaking of sensitive information).
Users can also explicitly specify '--diagnose=(stats|all)' to generate the
respective archive created by 'git diagnose --mode=(stats|all)'.

Suggested-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>