Revert more visibility changes in HistoryBlob.php
This follows-up
bebcf2c58381, which only fixed DiffHistoryBlob.
MediaWiki will fail to load some revisions if the following properties
do not unserialize correctly:
* HistoryBlobStub::$mOldId
* HistoryBlobStub::$mHash
* HistoryBlobCurStub::$mCurId
Visibility of HistoryBlobStub::$mRef is also restored; though the
property's value does not appear to be used after unserialization,
the property is nevertheless a serialized property.
Bug: 65665
Change-Id: I2724d15f9206d66d615d020117e60bb772048c19