!21542 Part 1: Add new getters for Web IDE store

What does this MR do?

• getDiffInfo determines if any diff exists in staged and unstaged changes for this path
• isFileDeletedAndReadded tells if a file is deleted in staged and readded in unstaged

These functions are used in !21676 (merged) and !21680 (merged).

This MR is the first part of the breakdown of MR !21542 (closed).

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