Limit push email diff size

What does this MR do?

Restrict the size of diffs in push emails to 30 lines / 150 KB (whichever is smaller), following https://gitlab.com/gitlab-org/gitlab_git/merge_requests/85.

Are there points in the code the reviewer needs to double check?

30 is a pretty arbitrary number - it's still possible to get an email of around 3 MB with this. We can increase or decrease it pretty easily.

Why was this MR needed?

Because emails on push can get very large if they contain a lot of files that sneak under the single-file limit.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/490

Screenshots (if relevant)

Byte limit:

Not hit limit:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together