rebase: calculate obsolescense-related info earlier

My goal is to use `rewriteutil.precheck()` in the rebase code. Since
rebase does its own handling of divergent commits (it skips them
instead of erroring out), we need to have divergence-causing commits
filtered out early. This patch helps prepare for that.

Differential Revision: https://phab.mercurial-scm.org/D10247

hgext/rebase.py

@@ -368,6 +368,8 b' class rebaseruntime(object):'
         self.resume = True
         try:
             self.restorestatus()
+            # Calculate self.obsoletenotrebased
+            self._handleskippingobsolete()
             self.collapsemsg = restorecollapsemsg(self.repo, isabort)
         except error.RepoLookupError:
             if isabort:
@@ -434,6 +436,9 b' class rebaseruntime(object):'
 
         self.prepared = True
 
+        # Calculate self.obsoletenotrebased
+        self._handleskippingobsolete()
+
     def _assignworkingcopy(self):
         if self.inmemory:
             from mercurial.context import overlayworkingctx
@@ -467,9 +472,6 b' class rebaseruntime(object):'
                             _(b'cannot collapse multiple named branches')
                         )
 
-        # Calculate self.obsoletenotrebased
-        self._handleskippingobsolete()
-
         # Keep track of the active bookmarks in order to reset them later
         self.activebookmark = self.activebookmark or repo._activebookmark
         if self.activebookmark: