[lldb/Editline] Fix mistake in HistoryOperation mapping

In 0e9b0b6d I introduced the
HistoryOperation enum to navigate the history. While this fixed the
behavior of HistoryOperation::Older and HistoryOperation::Newer, it
confused the mapping for HistoryOperation::Oldest and
HistoryOperation::Newest.

I tried to write a PExpect test to make sure this doesn't regress, but
I'm unable to prime the history in such a way that it recalls a known
element. I suspect this is an LLDB bug, but the  most recent entry
doesn't get update with entries from the current session. I considered
spoofing the home directory but that needs to happen before libLLDB is
loaded and you'll need to account for the widechar support. If anyone
has another suggestion I'd love to hear it.