Append album to singleton disambiguation string

- New config option for the importer 'singleton_album_disambig' lets
  users choose whether they want to display [album names] in the list of
  candidates. Enabled by default but ony applicable if the candidate provides
  an album attribute.
- Add docs describing the new option and which source plugins currently support
  it.

beets/config_default.yaml

@@ -34,6 +34,7 @@ import:
     bell: no
     set_fields: {}
     ignored_alias_types: []
+    singleton_album_disambig: yes
 
 clutter: ["Thumbs.DB", ".DS_Store"]
 ignore: [".*", "*~", "System Volume Information", "lost+found"]

beets/ui/commands.py

@@ -220,6 +220,9 @@ def disambig_string(info):
             disambig.append("Index {}".format(str(info.index)))
         if info.track_alt:
             disambig.append("Track {}".format(info.track_alt))
+        if (config['import']['singleton_album_disambig'].get()
+                and info.get('album')):
+            disambig.append("[{}]".format(info.album))
 
     if disambig:
         return ', '.join(disambig)

docs/reference/config.rst

@@ -760,6 +760,19 @@ Fields are persisted to the media files of each track.
 
 Default: ``{}`` (empty).
 
+.. _singleton_album_disambig:
+
+singleton_album_disambig
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+During singleton imports and if the metadata source provides it, album names
+are appended to the disambiguation string of matching track candidates. For
+example: ``The Artist - The Title (Discogs, Index 3, Track B1, [The Album]``.
+This feature is currently supported by the :doc:`/plugins/discogs` and the
+:doc:`/plugins/spotify`.
+
+Default: ``yes``.
+
 .. _musicbrainz-config:
 
 MusicBrainz Options