Display merged commit sha in fast-forward merge mode

What does this MR do?

Relate to #237902 (closed)

• Display merged commit sha in fast-forward merge mode. This feature helps visibility in identifying the commit related to the MR.
• merged_commit_sha should be set as follows:

Merge commit	Squash and fast-forward merge	Just fast-forward merge
merge_commit_sha	squash_commit_sha	diff_head_sha

Before

After

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