Add FF to avoid directly accessing diffs via gitaly

What does this MR do?

The Merge Request Changes API exposes the full diff by going directly to Gitaly, avoiding the existing DB-backed diffs (#225322 (comment 378507185)) This MR causes us to prefer the DB-backed diffs, but as those diffs are potentially limited to certain max number of files, bytes, etc, we use a feature flag to allow impacted projects who are willing to accept potentially non-performant requests to use the existing legacy approach that they had previously relied upon.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

  • [-] Label as security and @ mention @gitlab-com/gl-security/appsec
  • [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
  • [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #225322 (closed)

Edited by Mayra Cabrera