use _dirty.discard, not _dirty.remove

In the case where the field has not actually changed, don't throw a KeyError;
just proceed silently.

beets/ui/commands.py

@@ -920,7 +920,7 @@ def update_items(lib, query, album, move, pretend):
                     old_data['albumartist'] == old_data['artist'] == \
                         item.artist:
                 item.albumartist = old_data['albumartist']
-                item._dirty.remove('albumartist')
+                item._dirty.discard('albumartist')
 
             # Get and save metadata changes.
             changes = {}

docs/changelog.rst

@@ -47,6 +47,10 @@ And some fixes:
   "image.jpg not found during copy" error. Now clutter is removed (and
   directories pruned) much later in the process, after the
   ``import_task_files`` hook.
+* :doc:`/plugins/missing`: Fix an error when printing missing track names.
+  Thanks to Pedro Silva.
+* Fix an occasional KeyError in the :ref:`update-cmd` command introduced in
+  1.3.0.
 
 .. _Opus: http://www.opus-codec.org/
 .. _@Verrus: https://github.com/Verrus