Merge branch 'dd/help-autocorrect-never'

"git $cmd $args", when $cmd is not a recognised subcommand, by
default tries to see if $cmd is a typo of an existing subcommand
and optionally executes the corrected command if there is only one
possibility, depending on the setting of help.autocorrect; the
users can now disable the whole thing, including the cycles spent
to find a likely typo, by setting the configuration variable to
'never'.

* dd/help-autocorrect-never:
  help.c: help.autocorrect=never means "do not compute suggestions"