Show only ≤16 lines of codes in a discussion in a MR (issue #2860).

The code shown in a discussion will now be trimmed to include only the previous 16 lines or the closest '@@' line, whichever comes first (like GitHub). This allows user able to reach the discussion without scrolling through all irrelevant changes in the MR page.

This PR fixes #2860.

![screenshot](https://f.cloud.github.com/assets/103023/121115/6eaecde8-6d71-11e2-8e62-e1790508d733.png)