completion: fix multiple command removals

Commit 6532f3740b ("completion: allow to customize the completable
command list", 2018-05-20) tried to allow multiple space-separated
entries in completion.commands. To do this, it copies each parsed token
into a strbuf so that the result is NUL-terminated.

However, for tokens starting with "-", it accidentally passes the
original non-terminated string, meaning that only the final one worked.
Switch to using the strbuf.

Reported-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.c

@@ -386,8 +386,8 @@ void list_cmds_by_config(struct string_list *list)
 		const char *p = strchrnul(cmd_list, ' ');
 
 		strbuf_add(&sb, cmd_list, p - cmd_list);
-		if (*cmd_list == '-')
-			string_list_remove(list, cmd_list + 1, 0);
+		if (sb.buf[0] == '-')
+			string_list_remove(list, sb.buf + 1, 0);
 		else
 			string_list_insert(list, sb.buf);
 		strbuf_release(&sb);

t/t9902-completion.sh

@@ -1484,7 +1484,7 @@ test_expect_success 'git --help completion' '
 	test_completion "git --help core" "core-tutorial "
 '
 
-test_expect_failure 'completion.commands removes multiple commands' '
+test_expect_success 'completion.commands removes multiple commands' '
 	test_config completion.commands "-cherry -mergetool" &&
 	git --list-cmds=list-mainporcelain,list-complete,config >out &&
 	! grep -E "^(cherry|mergetool)$" out