revlog: recommit
49fd21f32695 with a fix for issue6528
`filelog.size` currently special cases two forms of metadata encoding:
- copy data via the parent order as flag bit
- censor data by peaking into the raw delta
All other forms of metadata encoding including the empty metadata block
are mishandled. In `basefilectx.cmp` the empty metadata block is
explicitly checked to compensate for this.
Restore 49fd21f32695, but disable it for filelog, so that the original
flag bit use contines to work. Document all this mess for now in
preparation of a proper rework.
Differential Revision:
https://phab.mercurial-scm.org/D11203
