Git: Save silently on undo actions

If the user wants to undo, first save the file silently. Then the user is
still able to cancel the undo operation.

Task-number: QTCREATORBUG-16647
Change-Id: I61ac42ff41f3f7758d662692bc92850766beab8e
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/git/gitplugin.cpp

@@ -718,7 +718,7 @@ void GitPlugin::logRepository()
 void GitPlugin::undoFileChanges(bool revertStaging)
 {
     if (IDocument *document = EditorManager::currentDocument()) {
-        if (!DocumentManager::saveModifiedDocument(document))
+        if (!DocumentManager::saveModifiedDocumentSilently(document))
             return;
     }
     const VcsBasePluginState state = currentState();