TaskTree: Rename optional into finishAllAndDone

Rationale:
1. More descriptive.
2. More consistent with a planned new finishAllAndError policy.
3. Limits the possibilities of making a conflict with std::optional.

Change-Id: I5155630188e4b699e6c18b13a101e0e2d4fe98f2
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/git/gitclient.cpp

@@ -465,11 +465,11 @@ ShowController::ShowController(IDocument *document, const QString &id)
         parallel,
         onGroupSetup([this] { setStartupFile(VcsBase::source(this->document()).toString()); }),
         Group {
-            optional,
+            finishAllAndDone,
             ProcessTask(setupDescription, onDescriptionDone),
             Group {
                 parallel,
-                optional,
+                finishAllAndDone,
                 onGroupSetup(desciptionDetailsSetup),
                 ProcessTask(setupBranches, onBranchesDone, onBranchesError),
                 ProcessTask(setupPrecedes, onPrecedesDone, onPrecedesError),