getRevisionInfo much faster (cached)

Makes getRevisionInfo much faster when loading all the revisions of a page.
This is done by efficiently parsing the 'changes.log' data and caching the
results (in memory) so that future calls to getRevisionInfo return
immediately without reloading the 'changes.log' file.

(Note: the changelog system in DokuWiki should still be rewritten so
that changes are not logged into one huge file!)

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