help tests: test --config-for-completion option & output

Add a regression test for the --config-for-completion option, this was
tested for indirectly with the test added in 7a09a8f093 (completion:
add tests for 'git config' completion, 2019-08-13), but let's do it
directly here as well.

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

t/t0012-help.sh

@@ -41,7 +41,8 @@ test_expect_success 'invalid usage' '
 	test_expect_code 129 git help -g add &&
 	test_expect_code 129 git help -a -g &&
 
-	test_expect_code 129 git help -g -c
+	test_expect_code 129 git help -g -c &&
+	test_expect_code 0 git help --config-for-completion add
 '
 
 test_expect_success "works for commands and guides by default" '
@@ -96,6 +97,20 @@ test_expect_success 'git help -c' '
 	test_cmp expect actual
 '
 
+test_expect_success 'git help --config-for-completion' '
+	git help -c >human &&
+	grep -E \
+	     -e "^[^.]+\.[^.]+$" \
+	     -e "^[^.]+\.[^.]+\.[^.]+$" human |
+	     sed -e "s/\*.*//" -e "s/<.*//" |
+	     sort -u >human.munged &&
+
+	git help --config-for-completion >vars &&
+	sort -u <vars >vars.new &&
+	mv vars.new vars &&
+	test_cmp human.munged vars
+'
+
 test_expect_success 'generate builtin list' '
 	git --list-cmds=builtins >builtins
 '