rebase: use `last` instead of direct indexing

This makes it compatible with all smartset classes.

hgext/rebase.py

@@ -596,7 +596,7 b' def nearestrebased(repo, rev, state):'
     rebased = [r for r in state if state[r] > nullmerge]
     candidates = repo.revs('max(%ld  and (::%d))', rebased, rev)
     if candidates:
-        return state[candidates[0]]
+        return state[candidates.first()]
     else:
         return None