Add old tiddler to th-saving-tiddler hook

Addresses https://github.com/Jermolene/TiddlyWiki5/pull/4434#issuecomment-645939182

core/modules/widgets/navigator.js

@@ -320,7 +320,7 @@ NavigatorWidget.prototype.handleSaveTiddlerEvent = function(event) {
 					"draft.title": undefined,
 					"draft.of": undefined
 				},this.wiki.getModificationFields());
-				newTiddler = $tw.hooks.invokeHook("th-saving-tiddler",newTiddler);
+				newTiddler = $tw.hooks.invokeHook("th-saving-tiddler",newTiddler,tiddler);
 				this.wiki.addTiddler(newTiddler);
 				// If enabled, relink references to renamed tiddler
 				var shouldRelink = this.getAttribute("relinkOnRename","no").toLowerCase().trim() === "yes";

editions/dev/tiddlers/new/th-saving-tiddler.tid

@@ -1,5 +1,5 @@
 created: 20150908150314994
-modified: 20170209145506427
+modified: 20200618114855781
 tags: HookMechanism
 title: Hook: th-saving-tiddler
 type: text/vnd.tiddlywiki
@@ -8,7 +8,8 @@ This hook allows plugins to inspect or modify tiddlers before they are saved via
 
 Hook function parameters:
 
-* ''tiddler'': tiddler object about to be saved
+* ''newTiddler'': tiddler object about to be saved
+* ''oldTiddler'': tiddler object of draft tiddler that is being saved (from v5.1.23)
 
 Return value: