How do I update a working tree to an old revision?

[Nicholas Allen]

Hi,

I would like to update my working tree to an old revision to see where a
bug was introduced. The update command does not have a -r option to
specify the revision. revert is not what I want because this reverts the
changes but keeps the working tree at the tip revision (so the changes
appear as changes to commit).

Cheers,

Nick