TaskTree: Introduce CallDoneIf enum

Get rid of CustomTask c'tor taking 3 handlers.
If the done handler needs to be called only on
success or an error, add explicit 3rd arg of CallDoneIf type.

Task-number: QTCREATORBUG-29834
Change-Id: I10e55415587e6cac46620dd5177ad8269584583c
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/subversion/subversionclient.cpp

@@ -205,7 +205,7 @@ SubversionDiffEditorController::SubversionDiffEditorController(IDocument *docume
             ProcessTask(onDescriptionSetup, onDescriptionDone)
         },
         Group {
-            ProcessTask(onDiffSetup, onDiffDone),
+            ProcessTask(onDiffSetup, onDiffDone, CallDoneIf::Success),
             postProcessTask(diffInputStorage)
         }
     };