WikiPage::doModify: Reuse old revision while null-editing

WikiPage::doModify: Reuse old revision while null-editing

The expected behavior for a null-edit is that the cache gets fully
purged as if another edit has happened, without actually inserting
a new revision into the database. Old implementation was to create
a new Revision instance, copy some of the private instance properties
from the old Revision instance, and skip database insertion. This
method is, unfortunately, prone to errors, as hooks and parsers
expect the given revision data to be equal to whatever the latest
revision is in the database, including timestamp and edit summary.

An alternative solution would be to maintain a method that copies
all the needed data from one revision to another; however, the extra
maintenance cost would be that it would be annoying to maintain it.
This implementation simply reuses the old Revision instance given
by the caller (WikiPage::doEditContent), as it is the latest
revision that hooks and parsers expect.

This patch fixes the error where, after null-editing, the
revision-related magic words, such as {{REVISIONDAY}}, are changed
to the values given by the null edit, instead of the latest revision.
Old behavior was caused by the parser parsing with incorrect
revision data.

Deprecate Revision::setUserIdAndName. It was a hack added in
147f79e for the same bug, addressing {{REVISIONUSER}}, but failed
to address the other magic words, including {{REVISIONDAY}}. I
failed to find any other usage of this instance method.

Bug: T135261
Change-Id: Ifce6a753effb98123574bff45ff59b5c9780f0c2