Request: non-exact size match mode - files may differ on tags only

- nonexact size match mode. it is pretty common for audio data (99% of filesize...) to be identical, but file sizes non-identical - when only tags/padding differ. it may be challenging to spot possible cases of this - size difference very small, or differing by a fixed amount could be a start. see 'whatmigrate' for ideas on how to treat them.