builtin/help.c: speed up is_git_command() by checking for builtin commands first

Since 2dce956 is_git_command() is a bit slow as it does file I/O in
the call to list_commands_in_dir(). Avoid the file I/O by adding an
early check for the builtin commands.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/help.c

@@ -288,6 +288,9 @@ static struct cmdnames main_cmds, other_cmds;
 
 static int is_git_command(const char *s)
 {
+	if (is_builtin(s))
+		return 1;
+
 	load_command_list("git-", &main_cmds, &other_cmds);
 	return is_in_cmdlist(&main_cmds, s) ||
 		is_in_cmdlist(&other_cmds, s);