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

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

The --all and --guides commands could be combined, which wouldn't have
any impact on the output except for:

    git help --all --guides --no-verbose

Listing the guide alongside that output was clearly not intended, so
let's error out here. See 002b726a400 (builtin/help.c: add
list_common_guides_help() function, 2013-04-02) for the initial
implementation.

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