Lazy diffs

Needs https://gitlab.com/gitlab-org/gitlab_git/merge_requests/65

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/10785

-   Use Gitlab::Git::DiffCollection which limits the result set size.
    This avoids loading unnecessary data from Git/SQL into memory.
-   Remove left-over Grit timeout handling code.
-   Use Gitlab::Git::Count.lines to count lines in strings.
-   Use more iterators.

See merge request !2705