Add link to compare changes intoduced by a git submodule update

What does this MR do?

This MR adds a link to the diff view of submodule commits so that the differences can be shown with a single click.

The link points to http://gdk.test:3000/Phaiax/trsub/-/compare/bcead1aaff41d4441f859772c9319ddf8c4664f1...ac5cacd1be252d3cb680e51274a1901a9c5d783f

Related issue: #24093

It would be nicer to include all changes or the commit messages, but that takes more effort.

Screenshots

Merge view (vue):

Commit diff view (haml)

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

The new compare link needs to be tested with github hosted submodules as well.

• 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