diff.c: fixup garding of config parser from value=NULL

Christian Couder noticed that there still were a handcrafted error()
call that we should have converted to config_error_nonbool() where
parse_lldiff_command() parses the configuration file.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -87,7 +87,7 @@ static int parse_lldiff_command(const char *var, const char *ep, const char *val
 	}
 
 	if (!value)
-		return error("%s: lacks value", var);
+		return config_error_nonbool(var);
 	drv->cmd = strdup(value);
 	return 0;
 }