drush hacked-revert [project], to download the unhacked original.

In combination with git, it would be useful to have a command that could revert a hacked project to its original contrib state, without updating it.
This would be equivalent to "drush up", but keeping the version, or "drush dl", automatically picking the current version.

The typical situation is where I have a partially "hacked" site in a git repository, and I want to create an "unpatched" branch as a parent of the patched branches. Then I can use the unpatched branch for contrib updates, before I merge them back into the main branches.
To get started with all this, I first need to revert all hacks.