This allows Revision::getRevisionText to get
a different BlobStore instance when $wiki is passed in
restoring the behaviour for $wiki before the MCR Revision
overhaul patch was merged.
Ia4c20a91e98df0b9b14b138eb4825c55e5200384
Bug: T183634
Bug: T183631
bug: T183583
Change-Id: Ib0949454e9a003c2965adc1aab38e31fcf121afe