usage: allow dying without writing an error message

Sometimes code wants to die in a situation where it already has written
an error message. To use the same error code as `die()` we have to use
`exit(128)`, which is easy to get wrong and leaves magic numbers all
over our codebase.

Teach `die_message_builtin()` to not print any error when passed a
`NULL` pointer as error string. Like this, such users can now call
`die(NULL)` to achieve the same result without any hardcoded error
codes.

Adapt a couple of builtins to use this new pattern to demonstrate that
there is a need for such a helper.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

usage.c

@@ -67,6 +67,8 @@ static NORETURN void usage_builtin(const char *err, va_list params)
 
 static void die_message_builtin(const char *err, va_list params)
 {
+	if (!err)
+		return;
 	trace2_cmd_error_va(err, params);
 	vreportf(_("fatal: "), err, params);
 }