Git: Separate internal plugin access API from plugin class

Plan is to hide the plugin class definition to the .cpp later.

Change-Id: I27f6d2dd23adb4f3ab47d99b0626956889d2750f
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/mergetool.cpp

@@ -225,7 +225,7 @@ void MergeTool::done()
 
     const FilePath workingDirectory = m_process.workingDirectory();
     gitClient().continueCommandIfNeeded(workingDirectory, success);
-    GitPlugin::emitRepositoryChanged(workingDirectory);
+    emitRepositoryChanged(workingDirectory);
     deleteLater();
 }