completion: add a GIT_COMPLETION_SHOW_ALL_COMMANDS

completion: add a GIT_COMPLETION_SHOW_ALL_COMMANDS

Add a GIT_COMPLETION_SHOW_ALL_COMMANDS=1 configuration setting to go
with the existing GIT_COMPLETION_SHOW_ALL=1 added in
c099f579b98 (completion: add GIT_COMPLETION_SHOW_ALL env var,
2020-08-19).

This will include plumbing commands such as "cat-file" in "git <TAB>"
and "git c<TAB>" completion. Without/with this I have 134 and 243
completion with git <TAB>, respectively.

It was already possible to do this by tweaking
GIT_TESTING_PORCELAIN_COMMAND_LIST= from the outside, that testing
variable was added in 84a97131065 (completion: let git provide the
completable command list, 2018-05-20). Doing this before loading
git-completion.bash worked:

    export GIT_TESTING_PORCELAIN_COMMAND_LIST="$(git --list-cmds=builtins,main,list-mainporcelain,others,nohelpers,alias,list-complete,config)"

But such testing variables are not meant to be used from the outside,
and we make no guarantees that those internal won't change. So let's
expose this as a dedicated configuration knob.

It would be better to teach --list-cmds=* a new category which would
include all of these groups, but that's a larger change that we can
leave for some other time.

1. https://lore.kernel.org/git/CAGP6POJ9gwp+t-eP3TPkivBLLbNb2+qj=61Mehcj=1BgrVOSLA@mail.gmail.com/

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