Do we need to copy the code if we're already requiring the diff-lcs gem?

If we do, at the top of this file, add a note and the terms of the MIT License below the existing license header and a note to the NOTICE file at the top of the tree.

diff.rb, below the existing copyright header:

    Some portions of this file are derived from material in the diff-lcs project licensed under the terms of the MIT license, provided below.

    Copyright:: Copyright (c) 2004–2013 Austin Ziegler
    License:: MIT

    Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files
    (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge,
    publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do
    so, subject to the following conditions:

    The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT
    NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND 
    NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, 
    DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT 
    OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Then in the NOTICE file:

    Ohai incorporates code modified from diff-lcs (http://diff-lcs.rubyforge.org/), which is Copyright (c) 2004–2013 Austin Ziegler