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.

hgext/rebase.py

@@ -18,7 +18,7 b' from mercurial import hg, util, repair, '
 from mercurial import extensions, patch, scmutil, phases, obsolete, error
 from mercurial import copies
 from mercurial.commands import templateopts
-from mercurial.node import nullrev
+from mercurial.node import nullrev, nullid, hex
 from mercurial.lock import release
 from mercurial.i18n import _
 import os, errno
@@ -737,8 +737,12 b' def storestatus(repo, originalwd, target'
     f.write('%s\n' % (activebookmark or ''))
     for d, v in state.iteritems():
         oldrev = repo[d].hex()
-        if v > nullmerge:
+        if v >= 0:
             newrev = repo[v].hex()
+        elif v == revtodo:
+            # To maintain format compatibility, we have to use nullid.
+            # Please do remove this special case when upgrading the format.
+            newrev = hex(nullid)
         else:
             newrev = v
         f.write("%s:%s\n" % (oldrev, newrev))
@@ -780,6 +784,9 b' def restorestatus(repo):'
                 oldrev, newrev = l.split(':')
                 if newrev in (str(nullmerge), str(revignored)):
                     state[repo[oldrev].rev()] = int(newrev)
+                elif newrev == nullid:
+                    state[repo[oldrev].rev()] = revtodo
+                    # Legacy compat special case
                 else:
                     state[repo[oldrev].rev()] = repo[newrev].rev()