The git plugin provides the following environment variables:

GIT_PREVIOUS_COMMIT: The hash of the commit last built on this branch, if any.

GIT_PREVIOUS_SUCCESSFUL_COMMIT: The hash of the commit last successfully built on this branch, if any.

It would be useful if the mercurial plugin provided the same information, e.g.

MERCURIAL_PREVIOUS_REVISION: Full ID of revision last built on this job.

MERCURIAL_PREVIOUS_SUCCESSFUL_REVISION: Full ID of revision last successfully built on this job.