Add a Vuex action that changes what `commit` the Diffs app is currently viewing

What does this MR do?

For #18140 (closed).

Adds a Vuex action that changes the current commit that the Diffs app is viewing.

Why

In order to load commit-by-commit information in the Diffs app, it needs some way to replace just portions of the state without triggering a complete page reload.

This action clears the currently loaded diff files (using the new SET_DIFF_FILES mutation), and swaps the endpoints to be used for fetching new data in the future to point to the new commit.

It then makes a request (via another action) for the diff metadata, which will as a matter of course overwrite the commit in state. Any interested components can observe this commit change and respond appropriately.

Screenshots

N/A, this is ~backstage work.

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