Mercurial Changelog should compare with previous build

Mercurial Changelog should compare with revision of the previous build instead of relying on local changes.
When using multiple slaves, current implementation is taking in account changes which were already tested by a previous build on a different slave.
Also, when a workspace is wiped out (or a new slave installed), changelog is empty since a full clone was needed.
That wanted behavior is already implemented by other SCM plugins.