history isn't shown if your system clock is set wrong

fish wants to use a single history file, but also ignore item from other sessions that were ran after this session was started. To achieve this it compares the item's timestamp against the current clock. The problem is that if the clock is invalid or set backwards, then old items may appear new and will be ignored.

The real problem is that fish ever re-reads the file. What ought to happen is that fish uses whatever is in the file at session startup or when `history merge` is run, and then ignores the timestamp.

Credit to @travankor for [pointing this out](https://github.com/fish-shell/fish-shell/pull/6493#issuecomment-586066717).