rebase: move state serialization to use unfiltered repo

Now that rebasestate is serialized as part of the transaction, the repo state it
sees is the version at the end of the transaction, which may have hidden nodes.
Therefore, it's possible parts of the rebase commit set are no longer visible by
the time the transaction is closing, which causes a filtered revision error in
this code. I don't think state serialization should be blocked from accessing
commits it knows exist, especially if all it's trying to do is get the hex of
them, so let's use an unfiltered repo here.

Unfortunately, the only known repro is with the fbamend Facebook extension, so
I'm not sure how to repro it in core Mercurial for a test.

hgext/rebase.py

@@ -169,7 +169,7 b' class rebaseruntime(object):'
                 self._writestatus(f)
 
     def _writestatus(self, f):
-        repo = self.repo
+        repo = self.repo.unfiltered()
         f.write(repo[self.originalwd].hex() + '\n')
         f.write(repo[self.target].hex() + '\n')
         f.write(repo[self.external].hex() + '\n')