mbsync: fix updating album with invalid first track MBID

MBID of recording could become invalid after merging. The existing
code always copies metadata from first track after updating. But for
albums with invalid track MBID that happens to be the first track,
MusicBrainz changes won't be applied to whole album, only whose
tracks with valid MBID. This is particularly annoying since those
changes are actually displayed for every `beet mbsync` run, but never
get applied.

Fix this issue by finding any track that get MusicBrainz updates, and
apply it to whole album.

beetsplug/mbsync.py

@@ -145,10 +145,13 @@ def albums(self, lib, query, move, pretend, write):
             with lib.transaction():
                 autotag.apply_metadata(album_info, mapping)
                 changed = False
+                # Find any changed item to apply MusicBrainz changes to album.
+                any_changed_item = items[0]
                 for item in items:
                     item_changed = ui.show_model_changes(item)
                     changed |= item_changed
                     if item_changed:
+                        any_changed_item = item
                         apply_item_changes(lib, item, move, pretend, write)
 
                 if not changed:
@@ -158,7 +161,7 @@ def albums(self, lib, query, move, pretend, write):
                 if not pretend:
                     # Update album structure to reflect an item in it.
                     for key in library.Album.item_keys:
-                        a[key] = items[0][key]
+                        a[key] = any_changed_item[key]
                     a.store()
 
                     # Move album art (and any inconsistent items).