Add file_identifier_hash to diff position

Merged Sincheol (David) Kim requested to merge use-file-identifier-hash-for-diff-files into master

What does this MR do?

This adds file_identifier_hash to GitLab::Diff::Position in order to use to find the correct diff_file. It's behind a feature flag and should not affect anything for now.

Related to #33867

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
Edited by Sincheol (David) Kim