Fix MR changes tab size count

What does this MR do?

When the number of files changed in an MR was greater than 100, the changes tab counter always showed '100' - until this change!

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

Don't think so.

Why was this MR needed?

I introduced this bug when fixing a performance issue in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8310. This shouldn't be a performance concern, as the problem was that by going through Gitlab::Diff::FileCollection::MergeRequestDiff, we were highlighting the diffs just to count them, which we don't need when we're not actually rendering the diffs anywhere.

Screenshots (if relevant)

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/27563.