Avoid "No activeWorkbenchWindow found" error when comparing commits

Using HandlerUtil.getActiveWorkbenchWindowChecked(event) is known to
have issues. Therefore, it is better to use the good old
PlatformUI.getWorkbench().getActiveWorkbenchWindow() for retrieving the
active workbench window.

Bug: 421444
Change-Id: I80cfef1ce9559eab23c60de273d9d076442dcba9
Signed-off-by: Kaloyan Raev <kaloyan.r@zend.com>