Switch from using random number to Git root's hash for tmpfile name

This fixes a bug with which we couldn't locate and use the previous
temp file if the rebase is interrupted and resumed later. On the
other hand, this shouldn't cause conflicts as you can't have two
simultaneous rebase operations in the same Git repo anyway.
Don't rewrite the temp file if we're resuming an interrupted rebase

Additionally, we detect the existence of the `--edit-todo` option and
fail if it exists. Also bumped version to 0.2.1.