revlog: simplify entry update logic in `rewrite_sidedata`
We don't need to rebuild the full entry. So lets just gather the data we need
to pass to `replace_sidedata_info`.
Differential Revision: https://phab.mercurial-scm.org/D10642
public attribute not declared in interfaces: badpeer.badattribute
public attribute not declared in interfaces: badpeer.badmethod