Pass 'pst' as true in parse request for fragments

In our parse request for 'wikitext fragments', used in the transclusion dialog
and extension inspectors, we weren't passing 'pst' as true, so while [[Foo|Bar]]
came back as expected, [[Foo (bar)|]] just came back as plain text, confusing
users.

Bug: 60998
Change-Id: I9931ad8034273ceb19c027d5035c63422cc0e570