Limit push email diff size

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.

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

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

See merge request !4566