TaskTree: Introduce DoneWith enum

This makes it possible to recognize the cancel state
when the task was automatically stopped because of task's
parent group workflow policy or when the user called TaskTree::stop().

This addresses the 2nd point in the master task below.

Task-number: QTCREATORBUG-28741
Task-number: QTCREATORBUG-29834
Change-Id: I2798b9ec1d2f1d667aff51ee0271a5a15a525dc1
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/git/branchmodel.cpp

@@ -429,8 +429,8 @@ void BranchModel::refresh(const FilePath &workingDirectory, ShowError showError)
     };
 
     const auto onForEachRefDone = [this, workingDirectory, showError](const Process &process,
-                                                                      bool success) {
-        if (!success) {
+                                                                      DoneWith result) {
+        if (result != DoneWith::Success) {
             if (showError == ShowError::No)
                 return;
             const QString message = Tr::tr("Cannot run \"%1\" in \"%2\": %3")