merging albums with --set fields overwrites whole album

### Problem

when importing a track with:
- for instance --set=title=whatever
- then making so we hit the "this album is already in the library" dialog and choosing merge

the --set=title=whatever flag ends up affecting all existing tracks in the album as well.

fairly long, self-contained MWE:

```bash
#!/usr/bin/env bash
set -eux
basedir=$(mktemp -d)

# create a standalone config
cat > $basedir/conf.yaml < $basedir/src.wav

# set the "file editor" to a script that sets the "album" and "albumartist" fields
export EDITOR="sed -E -i 's/album:.*\$/album: album/;s/albumartist:.*\$/albumartist: artist/'"

# import 'full album'
# eDit, Accept
beet -c $basedir/conf.yaml imp --set=title=title1 $basedir/src.wav <