Try to predict the rev_id when preparing edits

Try to predict the rev_id when preparing edits

During both the edit stash and first parse in on page save,
guess what the rev_id will be and use that instead of null.
Only reparse if it turns out to be wrong. This avoids extra
parsing on wikis that have low-medium traffic, and does not
cost much. The parsing that can be avoided is:
a) in doEditContent() by using the stash
b) in doEditUpdates() by using the doEditContent() result,
   whether that was able to use the stash or not itself

Also improved the parse operation logging in save paths.

Bug: T137900
Change-Id: Ic6faae70a78b4e223e4d3585cefd482c0fa00677