rebase: set `prepared = True` at very end of `_preparenewrebase()`

Once we've set `rebaseruntime.prepared = True`, `rebaseruntime.repo`
starts returning the unfiltered repo. That will make my next patch
break, because that patch moves the call to `rewriteutil.precheck()`
after the call to `_handleskippingobsolete()`, which current happens
after `prepared = True`. We therefore need to prepare by moving
`prepared = True` a bit later, after `_handleskippingobsolete()`. I
don't think that matters for that call.

Differential Revision: https://phab.mercurial-scm.org/D10257

hgext/rebase.py

@@ -447,11 +447,11 b' class rebaseruntime(object):'
             if dest.closesbranch() and not self.keepbranchesf:
                 self.ui.status(_(b'reopening closed branch head %s\n') % dest)
 
-        self.prepared = True
-
         # Calculate self.obsolete_* sets
         self._handleskippingobsolete()
 
+        self.prepared = True
+
     def _assignworkingcopy(self):
         if self.inmemory:
             from mercurial.context import overlayworkingctx
@@ -2192,7 +2192,6 b' def _compute_obsolete_sets(repo, rebaseo'
     obsolete_with_successor_in_destination = {}
     obsolete_with_successor_in_rebase_set = set()
 
-    assert repo.filtername is None
     cl = repo.changelog
     get_rev = cl.index.get_rev
     extinctrevs = set(repo.revs(b'extinct()'))