Resolve code smells in `getFiles` in Web IDE `ide/stores/actions/tree.js`

What does this MR do?

Make getFiles not assume it has branch available

Currently, the getFiles action assumes that the state has details available for the branch passed to it. It uses it to fetch its last commit ID. This may or may not be the case. The action should instead take another parameter ref to fetch commits by if necessary.

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

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

References

• Related to #36523