Gerrit: Display dependent changes as tree.

Add a depth member to GerritChange indicating the tree depth
and populate it by searches over the dependent changes.
Display the root changes sorted by date and add dependent changes
to their deph=1 parent, avoiding deeply nested items.

Change-Id: I66ccf392ce47514841c661820797e81996e9e5cc
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/gerrit/gerritmodel.h

@@ -70,7 +70,7 @@ public:
 class GerritChange
 {
 public:
-    GerritChange() : number(0) {}
+    GerritChange() : number(0), depth(-1) {}
 
     bool isValid() const { return number && !url.isEmpty() && !project.isEmpty(); }
     QString filterString() const;
@@ -89,6 +89,7 @@ public:
     QString status;
     QDateTime lastUpdated;
     GerritPatchSet currentPatchSet;
+    int depth;
 };
 
 typedef QSharedPointer<GerritChange> GerritChangePtr;