help: error if [-a|-g|-c] and [-i|-m|-w] are combined

help: error if [-a|-g|-c] and [-i|-m|-w] are combined

Add more sanity checking to "git help" usage by erroring out if these
man viewer options are combined with incompatible command-modes that
will never use these documentation viewers.

This continues the work started in d35d03cf93e (help: simplify by
moving to OPT_CMDMODE(), 2021-09-22) of adding more sanity checking to
"git help". Doing this allows us to clarify the "SYNOPSIS" in the
documentation, and the "git help -h" output.

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