Fix diff syntax highlighting

### What does this MR do?

This MR fixes a syntax highlighting issue that that manifested itself when a user tried the following:

```
+aaa
+bbb
 ccc
```

However, a regression in 0abe98f0 caused the last line to be put on the second to last line:

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/a16515499d5e3fd9737370d89c73f671/image.png)

Instead of this:

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/59eca6efd4c290aa633b9fb54c98a70e/image.png)

This bug occurred because of some error in the logic of handling new lines in the content. Refactored HTML parser to avoid duplication of newline parsing.

### What are the relevant issue numbers?

Closes #2235

See merge request !1126