The following code takes a before and after string and creates a relative diff syntax which can edit the before string into the after. It has 3 operations <add>, <del>, and <del><add>.
x<add>string adds the given string after the xth line in the before. x<del> deletes the xth line in the before. x<del><add>string replaces the xth line in the before wiht the given string. All line indexing starts at 1.
There is a special edge case where the after is identical to the before, except that it has additional lines prepended to it. This requires a 0<add>string case which adds the string before any lines in the before
Fix `create_rel_diff` so that it can properly deal with this case. 