Fix the fragment of old-style links to forms
Old edit summaries may contain links like [[L1#L1-F1]] when mentioning
forms. To link to the new form of such links instead ([[L1#F1]]),
implement the EntityLinkFormatter::getFragment() method accordingly.
This requires Wikibase change I99d03e1654 to work properly, but can be
merged without it, hence the lack of Depends-On on this change.