Both `Array#reject` and `Array#include?` have linear time
complexity. This results in quadratic time complexity when
`Array#include?` is called within `Array#reject`'s block. Using
`Array`'s difference operator gives the same result in linear time.
In a site with ~14,000 files, this drops the time taken by some calls to
`SourceWatcher::poll_once!` from ~15 seconds each to just a few
milliseconds.
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