rebase: handle revtodo as a special value when storing/restoring state Revtodo happens to share its value with nullrev, but this is an implementation details, so we move away from it. After this changeset one can successfully change the values for all the constants and the tests still pass, but doing so would require more refactoring if we want to avoid breaking backward compatibility on the state file.

rebase: add a 'revtodo' constant The state mapping is using '-1' to mark revisions that have not been rebased yet. We introduce and use a constant for that purpose. This will help emphasize the fact the value means something other than nullrev.

rebase: use '>= 0' to know is a revision was rebased The fact that the state for "not yet rebased" is -1 is an implementation details. So we change the comparisons to some semantically more correct.