Improve output of SideBySideDiff

When comparing two files and showing a side by side diff (for example in the html output), there are some cases where the compared lines do not match.

For example, when comparing file1.txt and file2.txt, here's what the output currently looks like:

file1.txt	file2.txt
This ​is ​a ​text file	This is another text file
It contains a few lines	It contains a few lines
But nothing to fancy	I added a line
	But nothing too fancy

Here's how it looks with the changes from this MR:

file1.txt	file2.txt
This ​is ​a ​text file	This is another text file
It contains a few lines	It contains a few lines
	I added a line
But nothing to fancy	But nothing too fancy

This may not seem like much, but sometimes diffs become unreadable because the wrong lines are being compared together.