mk-origtargz: Honor Files-ExcludeIgnored stanza

A common use-case for this can be seen in golang packages where one wants to exclude most but not all subdirectories under the "vendor/" directory. Many upstream packages aggressivly vendor dependencies, most of which are already included in debian. In some cases, it is not worth the trouble to package them seperately, in which case the new Files-ExcludeIgnored stanza can be very convenient to leave those sources in.

Closes: #829646