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
Merge "Revert more visibility changes in HistoryBlob.php"