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:

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/16e513894f7ecda1b111fe7d43e7f388/image.png)

With this fix, it looks like:

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/90fd635a937a68f1b2403740ebc75e3a/image.png)

Closes #2253

See merge request !1143