Fix T50385: Deadlock in BKE_libblock_remap_locked.

Am pretty sure node update should not touch to Main database like that,
but for now let's allow it, I guess the hack is needed for things like
Sverchok. ;)

source/blender/blenkernel/intern/library_remap.c

@@ -562,8 +562,14 @@ void BKE_libblock_remap_locked(
 		default:
 			break;
 	}
+
 	/* Node trees may virtually use any kind of data-block... */
+	/* XXX Yuck!!!! nodetree update can do pretty much any thing when talking about py nodes,
+	 *     including creating new data-blocks (see T50385), so we need to unlock main here. :(
+	 *     Why can't we have re-entrent locks? */
+	BKE_main_unlock(bmain);
 	libblock_remap_data_postprocess_nodetree_update(bmain, new_id);
+	BKE_main_lock(bmain);
 
 	/* Full rebuild of DAG! */
 	DAG_relations_tag_update(bmain);