config: report a bug if git_dir exists without commondir

This did happen at some stage, and was fixed relatively quickly. Make
sure that we detect very quickly, too, should that happen again.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -1668,6 +1668,8 @@ static int do_git_config_sequence(const struct config_options *opts,
 	if (opts->commondir)
 		repo_config = mkpathdup("%s/config", opts->commondir);
+	else if (opts->git_dir)
+		BUG("git_dir without commondir");
 	else
 		repo_config = NULL;