(#9671) Use Array#concat rather than +=

+= reassigns the variable it's used on, which means it creates a new
copy of the array each time, which is slow and wasteful. Array#concat,
on the other hand, modifies the object to which the variable refers,
requiring no copy and leaving no orphaned array, and so is much, much
faster.

Paired-With: Josh Cooper <josh@puppetlabs.com>