Git: Modernize

* pragma once
* member initialization
* s/struct/class/
* Introduce a static GitPlugin::client() method and use it

Change-Id: Ifdcac86dd16f3cdba11d564d03e9a15f00a6afdb
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/branchmodel.cpp

@@ -193,8 +193,7 @@ public:
 BranchModel::BranchModel(GitClient *client, QObject *parent) :
     QAbstractItemModel(parent),
     m_client(client),
-    m_rootNode(new BranchNode),
-    m_currentBranch(0)
+    m_rootNode(new BranchNode)
 {
     QTC_CHECK(m_client);