rebase: remove unnecessary and incorrect handling of nullid

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

--- a/hgext/rebase.py	Thu Apr 05 10:13:45 2018 -0700
+++ b/hgext/rebase.py	Thu Apr 05 11:01:42 2018 -0700
@@ -21,7 +21,6 @@
 
 from mercurial.i18n import _
 from mercurial.node import (
-    nullid,
     nullrev,
     short,
 )
@@ -252,7 +251,7 @@
                     else:
                         destrev = legacydest
                     destmap[oldrev] = destrev
-                    if newrev in (nullid, revtodostr):
+                    if newrev == revtodostr:
                         state[oldrev] = revtodo
                         # Legacy compat special case
                     else: