rollback: detect "parentgone" case earlier

Detecting this earlier will help us to affect the rollback process sooner, which
will help the next changesets.

To keep things simple, we degrade the behavior a bit when the `undo.desc` is
missing. However since `hg rollback` is not supposed to be used and the
`undo.desc` file have been introduced in mercurial 1.6 (2010). I think this is
an acceptable evil.

mercurial/localrepo.py

@@ -2709,6 +2709,8 b' class localrepository:'
     @unfilteredmethod  # Until we get smarter cache management
     def _rollback(self, dryrun, force, dsguard):
         ui = self.ui
+
+        parents = self.dirstate.parents()
         try:
             args = self.vfs.read(b'undo.desc').splitlines()
             (oldlen, desc, detail) = (int(args[0]), args[1], None)
@@ -2725,9 +2727,11 b' class localrepository:'
                 msg = _(
                     b'repository tip rolled back to revision %d (undo %s)\n'
                 ) % (oldtip, desc)
+            parentgone = any(self[p].rev() > oldtip for p in parents)
         except IOError:
             msg = _(b'rolling back unknown transaction\n')
             desc = None
+            parentgone = True
 
         if not force and self[b'.'] != self[b'tip'] and desc == b'commit':
             raise error.Abort(
@@ -2742,7 +2746,6 b' class localrepository:'
         if dryrun:
             return 0
 
-        parents = self.dirstate.parents()
         self.destroying()
         vfsmap = {b'plain': self.vfs, b'': self.svfs}
         transaction.rollback(
@@ -2757,8 +2760,6 b' class localrepository:'
             self.svfs.rename(b'undo.phaseroots', b'phaseroots', checkambig=True)
         self.invalidate()
 
-        has_node = self.changelog.index.has_node
-        parentgone = any(not has_node(p) for p in parents)
         if parentgone:
             # prevent dirstateguard from overwriting already restored one
             dsguard.close()

tests/test-rollback.t

@@ -200,6 +200,7 b' same again, but emulate an old client th'
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg rollback
   rolling back unknown transaction
+  working directory now based on revision 0
   $ cat a
   a