Two things here. One is to use `WebHistory._new_history` only as a to-save
queue, so we now add entries to `_old_urls` when they are first created and
can now no longer iterate of `_new_history` in `__iter__()`.
Second is to stop blindly tacking new history entries on the end of the
history completion model. It does involve iterating over the model to find the
existing entry but we only do that if we know the duplicate is there, which is
fast to check.
This also ads another point of mutation to the history completion model which
may prove problematic if it leads to more segfaults.
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