Prevent revisions with rev_page = 0 from being inserted into the DB

Prevent revisions with rev_page = 0 from being inserted into the DB

There's no good reason to ever do this, or to write code that lets it
happen. The revisions are just going to sit around in the DB, not
connected to anything (except maybe in some page's page_latest field,
if you're lucky). Any operations that do this should fail fast instead
of spamming the DB and appearing to "succeed".

Bug: T18674
Change-Id: I8219153a09adb1c556d0159a4fb0799895370a94