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
c099f579b9 (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 84a9713106 (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>

contrib/completion/git-completion.bash

@@ -49,6 +49,11 @@
 #     and git-switch completion (e.g., completing "foo" when "origin/foo"
 #     exists).
 #
+#   GIT_COMPLETION_SHOW_ALL_COMMANDS
+#
+#     When set to "1" suggest all commands, including plumbing commands
+#     which are hidden by default (e.g. "cat-file" on "git ca<TAB>").
+#
 #   GIT_COMPLETION_SHOW_ALL
 #
 #     When set to "1" suggest all options, including options which are
@@ -3455,7 +3460,13 @@ __git_main ()
 			then
 				__gitcomp "$GIT_TESTING_PORCELAIN_COMMAND_LIST"
 			else
-				__gitcomp "$(__git --list-cmds=list-mainporcelain,others,nohelpers,alias,list-complete,config)"
+				local list_cmds=list-mainporcelain,others,nohelpers,alias,list-complete,config
+
+				if test "${GIT_COMPLETION_SHOW_ALL_COMMANDS-}" = "1"
+				then
+					list_cmds=builtins,$list_cmds
+				fi
+				__gitcomp "$(__git --list-cmds=$list_cmds)"
 			fi
 			;;
 		esac

t/t9902-completion.sh

@@ -2440,6 +2440,37 @@ test_expect_success 'option aliases are shown with GIT_COMPLETION_SHOW_ALL' '
 	)
 '
 
+test_expect_success 'plumbing commands are excluded without GIT_COMPLETION_SHOW_ALL_COMMANDS' '
+	(
+		. "$GIT_BUILD_DIR/contrib/completion/git-completion.bash" &&
+		sane_unset GIT_TESTING_PORCELAIN_COMMAND_LIST &&
+
+		# Just mainporcelain, not plumbing commands
+		run_completion "git c" &&
+		grep checkout out &&
+		! grep cat-file out
+	)
+'
+
+test_expect_success 'all commands are shown with GIT_COMPLETION_SHOW_ALL_COMMANDS (also main non-builtin)' '
+	(
+		. "$GIT_BUILD_DIR/contrib/completion/git-completion.bash" &&
+		GIT_COMPLETION_SHOW_ALL_COMMANDS=1 &&
+		export GIT_COMPLETION_SHOW_ALL_COMMANDS &&
+		sane_unset GIT_TESTING_PORCELAIN_COMMAND_LIST &&
+
+		# Both mainporcelain and plumbing commands
+		run_completion "git c" &&
+		grep checkout out &&
+		grep cat-file out &&
+
+		# Check "gitk", a "main" command, but not a built-in + more plumbing
+		run_completion "git g" &&
+		grep gitk out &&
+		grep get-tar-commit-id out
+	)
+'
+
 test_expect_success '__git_complete' '
 	unset -f __git_wrap__git_main &&