VCS: Replace AbstractCheckoutJob with Command

Command now provides the same functionality. Deduplicate code.

Change-Id: I789f021050471281870b6ef6a81a94e66fbdf0c7
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/git/gitclient.h

@@ -33,7 +33,6 @@
 #include "gitsettings.h"
 
 #include <coreplugin/editormanager/ieditor.h>
-#include <vcsbase/command.h>
 
 #include <QObject>
 #include <QString>
@@ -52,8 +51,9 @@ namespace Core {
 }
 
 namespace VcsBase {
-    class VcsBaseEditorWidget;
+    class Command;
     class SubmitFileModel;
+    class VcsBaseEditorWidget;
 }
 
 namespace Utils {