Use current revision callback instead of template callback

The current revision callback will catch things like Scribunto's
getContent() and the REVISIONUSER/REVISIONTIMESTAMP parser functions. Use
it in place of the template callback, and also avoid duplicating a lot of
logic from statelessFetchTemplate.

Bug: 70495
Change-Id: Idefaa89e665a0caf7b47b6f67649b6a87b524ff4