WIP: Use commit SHA instead of branch id to request IDE files and contents

What does this MR do?

This MR uses the commit SHA instead of the branch ID to request data for the file tree and raw files in the IDE. The SHA is obtained as the most recent commit in the branch at the moment the branch is first loaded, then remains the same until the branch is changed or the window is refreshed. This is done to prevent unusual file behavior that could occur if another user edits the files while the IDE is open.

Closes #63160 (moved)

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

Performance 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