Draft: Add straight diff in the UI

What does this MR do?

Add straight diff to WebUI. This is usefull to show the differences between two branches.

Current Compare view is equivalent to git diff branch_a...branch_b (Changes that occurred on the branch_b branch since when the branch_a branch was started off it), this new view is equivalent to git diff branch_a..branch_b (Changes between the tips of the branch_a and the branch_b branches).

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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