...and hide errors relating to setting already-readonly variables.
`plugin/history-eternal` does not need to force loading after `plugin/history` because both plugins will play nicely with read-only variables, and since we're overwritting and marking read-only then the intended result survives no matter which loads first.
plugin/history-eternal: require Bash v4.3+
Unlimited history is only possible in _Bash_ version 4.3 and up
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