scalar-diagnose: use 'git diagnose --mode=all'

scalar-diagnose: use 'git diagnose --mode=all'

Replace implementation of 'scalar diagnose' with an internal invocation of
'git diagnose --mode=all'. This simplifies the implementation of
'cmd_diagnose' by making it a direct alias of 'git diagnose' and removes
some code in 'scalar.c' that is duplicated in 'builtin/diagnose.c'. The
simplicity of the alias also sets up a clean deprecation path for 'scalar
diagnose' (in favor of 'git diagnose'), if that is desired in the future.

This introduces one minor change to the output of 'scalar diagnose', which
is that the prefix of the created zip archive is changed from 'scalar_' to
'git-diagnostics-'.

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