Media: Properly reset the "add to gallery" library to ensure it accurately reflects the current gallery. see #22815.

git-svn-id: https://develop.svn.wordpress.org/trunk@23263 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/js/media-views.js

@@ -661,6 +661,10 @@
 				return !! this.mirroring.getByCid( attachment.cid ) && ! edit.getByCid( attachment.cid ) && media.model.Selection.prototype.validator.apply( this, arguments );
 			};
 
+			// Reset the library to ensure that all attachments are re-added
+			// to the collection. Do so silently, as calling `observe` will
+			// trigger the `reset` event.
+			library.reset( library.mirroring.models, { silent: true });
 			library.observe( edit );
 			this.editLibrary = edit;