Fix bug where backslashes in inline diffs could be dropped

This MR fixes a bug in inline diff generation causing backslashes to be dropped. For example, the input:

input.to_s.sub(/[\r\n].+/,'').sub(/\\[rn].+/, '').strip

The second backslash is dropped in the second sub statement:

With this fix, it looks like:

Closes #2253 (closed)