Stop "prefixing" up/down arrow history search

[vicnett]

What

Stop matching history entries that don't start with the beginning of the search term.
For example, ls should match ls -l but not echo ls.

Why

For one, I'm used to it being this way. I also think it makes more sense this way. The only exception I can think of right now is sudo but, again, I'm used to how it works and I don't think that's an issue... If a command needs sudo I'll probably have sudo <command> typed before trying to look at what I typed last time I used that command.

How

The zsh-history-substring documentation explains that the HISTORY_SUBSTRING_SEARCH_PREFIXED variable is checked for being empty or non-empty, and it being non-empty results in the wanted behavior. The actual value doesn't matter so I just made it 'foo' 😄