Bug fix 27164 image cannot be collapsed on merge request changes tab

What does this MR do?

This MR does solve this issue #27164 (closed).

• The issue was that hasDiffLines is returning undefined when given a non-readable file as input.
• The issue was solved and the method was renamed since an non-readable file doesn’t have any lines to speak of, I felt like isDiffable is a more appropriate method Name now (up for debate).
• A Testcase has been added including additional mock-data

Screenshots

Screen_Recording_2019-10-23_at_17.53.58

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

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