Fix newline bug in getDiffedLines

[andrewpareles]

We define a "diff" as a single green/red pair that denotes a change.

We create diffs by using the function getDiffedLines, which takes in an old string and the new string and outputs the information of each diff (the range of the diff, the original text of the diff, etc).
Right now, getDiffedLines occasionally outputs the wrong number of newlines. We think this caused by inconsistencies in diffLines:

Here is an example of the bug:

[andrewpareles]

This is really part of #7.