DiffEditor: When jumping to a file then put that block to the top

When jumping to a file via the dropdown box: Have that block be
scrolled to the top of the view, not to the center. This is way
less confusing.

Change-Id: I5c629dda1af8bff76ca93a70357aa35c8efcb37a
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/diffeditor/unifieddiffeditorwidget.cpp

@@ -37,6 +37,7 @@
 #include <QPlainTextEdit>
 #include <QMenu>
 #include <QPlainTextDocumentLayout>
+#include <QScrollBar>
 #include <QTextBlock>
 #include <QTextCodec>
 #include <QPainter>
@@ -734,7 +735,7 @@ void UnifiedDiffEditorWidget::setCurrentDiffFileIndex(int diffFileIndex)
     QTextCursor cursor = textCursor();
     cursor.setPosition(block.position());
     setTextCursor(cursor);
-    centerCursor();
+    verticalScrollBar()->setValue(blockNumber);
     m_ignoreCurrentIndexChange = oldIgnore;
 }