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/gitversioncontrol.h

@@ -23,8 +23,7 @@
 **
 ****************************************************************************/
 
-#ifndef GITVERSIONCONTROL_H
-#define GITVERSIONCONTROL_H
+#pragma once
 
 #include <coreplugin/iversioncontrol.h>
 
@@ -73,5 +72,3 @@ private:
 
 } // Internal
 } // Git
-
-#endif // GITVERSIONCONTROL_H