title: hg rebase abort fails: "unknown revision"
body: <p>Occasionally when performing a rebase using the MercurialEclipse plugin my repository gets thrown into an odd state.   It will become stuck in a "rebasing" state, yet when I try to abort the rebase (e.g. "hg rebase -a") I get this error message:</p>
  
  <blockquote>
    <p>abort: unknown revision '<em>xxxx</em>'! </p>
  </blockquote>
  
  <p>Where xxxx is a revision hash. Usually at this point I've abandoned all hope of performing the rebase -- I just want to get back to a happier time when my repository was not screwed up.  So far my solution has been to nuke my project directory and clone it again. Is there a less drastic solution?</p>