Conflicting `diff_hard_limit_enabled?` in emails_helper and diff_helper breaks full diff loads

diff_hard_limit_enabled is defined in two places now as a result of 117e6d49:

1. emails_helper.rb
2. diff_helper.rb

The override seems to returns false, and so MRs like this will never show up properly. For example:

https://gitlab.com/gitlab-org/gitlab-test/merge_requests/7/diffs?force_show_diff=true

Perhaps it's better to pass a local instead?

Relates to gitlab-org/gitlab-ce#13759

@stanhu Thanks for digging into this! I guess that's the downside of Rails's way of loading all the helpers and mixing them all in a big bag of helpers! :)

Added ~91797 label

Added regression label

Milestone changed to 8.6

Reassigned to @stanhu

mentioned in merge request !312 (closed)

mentioned in commit 564e8564

mentioned in merge request !313 (merged)

Status changed to closed by commit 564e85646d042f47cdb91e62b51dc068a4b1f568

mentioned in commit ea1f4c38

mentioned in commit 95d2264f

Mentioned in commit 564e8564