help.c: help.autocorrect=never means "do not compute suggestions"

help.c: help.autocorrect=never means "do not compute suggestions"

While help.autocorrect can be set to 0 to decline auto-execution of
possibly mistyped commands, it still spends cycles to compute the
suggestions, and it wastes screen real estate.

Update help.autocorrect to accept the string "never" to just exit
with error upon mistyped commands to help users who prefer to never
see suggested corrections at all.

While at it, introduce "immediate" as a more readable way to
immediately execute the auto-corrected command, which can be done
with negative value.

Signed-off-by: Drew DeVault <sir@cmpwn.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>