[PATCH 0 of 2] ensure we can abort from a stale rebase

[Jason Harris]

After doing a rebase which fails (due to conflicts, etc), and then removing the
changesets the rebase referenced, we cannot later exit from the rebase by using
rebase --abort.  That is, while rebase is setting up to do the abort of the
previous rebase, an exception is raised since it can't find the previous
changesets in the repo and so the rebase --abort terminates before resetting the
rebasestate.  Repeated uses of rebase --abort will not get around this.