Basically, I would like to revert a central SVN repository back to a previous commit, kinda like this:
svn update
svn merge -r 150:140 .
svn commit -m "Rolled back to r140"
But I'm using git-svn and cannot seem to find an equivalent. 
I've tried setting everything up locally on a new branch by checking out an earlier commit that was an ancestor of trunk (the current commit of the central repo). I made a few additional changes and now I cannot git svn rebase or git svn dcommit without getting an error from SVN.
I tried checking out that same commit without making the additional changes and then running git svn dcommit, this says it is committing to the central repo but doesn't actually seem to do anything.
How do I go back to a previous commit and then start making new changes?