builtin/diagnose.c: add '--mode' option

builtin/diagnose.c: add '--mode' option

Create '--mode=<mode>' option in 'git diagnose' to allow users to optionally
select non-default diagnostic information to include in the output archive.
Additionally, document the currently-available modes, emphasizing the
importance of not sharing a '--mode=all' archive publicly due to the
presence of sensitive information.

Note that the option parsing callback - 'option_parse_diagnose()' - is added
to 'diagnose.c' rather than 'builtin/diagnose.c' so that it may be reused in
future callers configuring a diagnostics archive.

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