TaskTree: Introduce For element

Improve code readability by requiring iteratable Group
to be named For.

The For's c'tor requires an iterator element as a first arg.

The For's c'tor allows for passing exactly one iterator element.

It's not possible to place iterators inside Group element anymore.

Change-Id: I9dfe2c0da058abac161f66c4e336da2417c383f1
Reviewed-by: hjk <hjk@qt.io>

src/plugins/git/gitclient.cpp

@@ -506,10 +506,10 @@ ShowController::ShowController(IDocument *document, const QString &id)
             updateDescription(*data);
         };
 
-        const Group recipe {
+        const For recipe {
+            iterator,
             parallel,
             continueOnSuccess,
-            iterator,
             ProcessTask(onFollowSetup, onFollowDone, CallDoneIf::Success),
             onGroupDone(onDone, CallDoneIf::Error)
         };