rebase: remove unnecessary and incorrect handling of nullid
We used to write a nullid as hex to the rebase state file and
interpret it as the "todo" state (value -1). However, when reading it,
we compared the string value to (binary) nullid, which would of course
not match. AFAICT, it still worked because when the read nodeid did
not match nullid (which, again, it didn't), we'd use the normal path
which did repo[<hex nullid>].rev(), and that also happens to return
-1. It seems to have been this way ever since 9972758ab4c5 (rebase:
handle revtodo as a special value when storing/restoring state,
2014-12-02).
Differential Revision: https://phab.mercurial-scm.org/D3140