(bug 15853) Wrong entries in history feed

(bug 15853) Wrong entries in history feed

The history feed for a non-existing page fetched revisions for
rev_page=0. Even though there should be none in the database,
there obviously are, which means such feeds are full of spurious
entries.

Added an existence check, fetchRevisions now returns an empty result
for non-existent pages, which is (and has always been) handled
gracefully in feed() by emitting feedEmpty().

Change-Id: Iae892d900239030bc249c829faecb1bcb3672db2