first, create a new branch in the current position (in case you need your old 'screwed up' history):
git branch fubar-pin
update your list of remote branches and sync new commits:
git fetch --all
then, reset your branch to the point where origin/branch points to:
git reset --hard origin/branch
be careful, this will remove any changes from your working tree!
~ Answered on 2011-06-03 16:20:39