In `write` and `update` previews, clarify which "side" is which

It would be nice to have a command for checking whether the beets db and the files are in sync.

I know, there's update and write with --pretend, but blindly entering either of those makes me feel uneasy every time. Additionally those leave a bit to be desired when it comes to representing which value is in the file and which is in the db.

Start by reading the implementations of the write and update commands and running both with --pretend to inspect their current previews. Determine how the output can clearly distinguish values from the files and the database, and whether a separate synchronization-check command is also required. Done means the preview labels are unambiguous and the requested sync check has defined behavior.

Written by the indexing model from the issue text.