Fix "Discard" for newly-created and renamed files

What does this MR do?

Clicking "Discard" button for newly created and renamed files results in the changes not being discarded correctly. Also a console error is seen. This MR fixes it.

Issue #27244 (closed) is about "Discard all", but "Discard" was also broken for new and renamed files, so I fixed it too.

However, as per #27242 (closed), "Discard" button in Web IDE is completely broken, which is fixed in !21902 (merged). So until !21902 (merged) is merged, this issue cannot be tested locally on GDK.

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

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