help: add --no-[external-commands|aliases] for use with --all

help: add --no-[external-commands|aliases] for use with --all

Add the ability to only emit git's own usage information under
--all. This also allows us to extend the "test_section_spacing" tests
added in a preceding commit to test "git help --all"
output.

Previously we could not do that, as the tests might find a git-*
command in the "$PATH", which would make the output differ from one
setup to another.

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