always show diff link if available

This patch makes a diff link appear in Special:Abuselog no matter a filter
is public or private. Because there is much information to figure out which
revision a filter catches, showing a diff link will make patrollers more
convenient without leaking more information.

Bug: 59695
Change-Id: I6a4432cbb41ae78583cc87355514f252984c1005
Merge "always show diff link if available"