revlog: use a `reading` context in `_enforceinlinesize`
We are about to enforce reading context on various operation, so we make sure
top level method are in the right context.
In the future we might move the responsibility of opening the revlog for reading
higher in the call chain but lets limit the disruption for now.
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
Mercurial Distributed SCM (version *) (glob)