Revert "FIX: When mutating a string to build a diff. Dup it first"

[romanrizzi]

Reverts #7482

This change should have been applied to the frozen_string_literal branch.

[discoursebot]

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[SamSaffron]

this fix itself does look a bit awkward, this file was by a mile the most complex one to convert to froze_string_literal: true, so much mutating of inputs going on.

[romanrizzi]

That is the only method in that class that mutates a string. Since String#insert is really handy, I thought using dup was the best option here.

I also applied this commit against frozen_string_literal but looks like it disappeared from history after a rebase. If this commit hits that branch, revisions will no longer work on dev.