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>