help tests: add test for --config output

Add a missing test for checking what the --config output added in
ac68a93fd2 (help: add --config to list all available config,
2018-05-26) looks like. We should not be emitting anything except
config variables and the brief usage information at the end here.

The second test regexp here might not match three-level variables in
general, as their second level could contain ".", but in this case
we're always emitting what we extract from the documentation, so it's
all strings like:

    foo.<name>.bar

If we did introduce something like variable example content here we'd
like this to break, since we'd then be likely to break the
git-completion.bash.

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

t/t0012-help.sh

@@ -77,6 +77,19 @@ test_expect_success 'git help -g' '
 	test_i18ngrep "^   tutorial   " help.output
 '
 
+test_expect_success 'git help -c' '
+	git help -c >help.output &&
+	cat >expect <<-\EOF &&
+
+	'\''git help config'\'' for more information
+	EOF
+	grep -v -E \
+		-e "^[^.]+\.[^.]+$" \
+		-e "^[^.]+\.[^.]+\.[^.]+$" \
+		help.output >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'generate builtin list' '
 	git --list-cmds=builtins >builtins
 '