help: correct logic error in combining --all and --config

help: correct logic error in combining --all and --config

Fix a bug in the --config option that's been there ever since its
introduction in 3ac68a93fd2 (help: add --config to list all available
config, 2018-05-26). Die when --all and --config are combined,
combining them doesn't make sense.

The code for the --config option when combined with an earlier
refactoring done to support the --guide option in
65f98358c0c (builtin/help.c: add --guide option, 2013-04-02) would
cause us to take the "--all" branch early and ignore the --config
option.

Let's instead list these as incompatible, both in the synopsis and
help output, and enforce it in the code itself.

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