This attribute is only used by LFS. It is used by one of the revlog
flag processor functions, which gets an instance of the revlog - not
the file storage type. So, it makes sense to store this attribute on
the revlog instead of the filelog.
With this change, I'm pretty confident that LFS is no longer directly
accessing file storage interface members that are revlog centric. i.e.
it gets us one step closer to eliminating revlog-centric APIs from the
file storage interface!