Remove unnecessary waiting in the diff file component

What does this MR do?

Removes some async/await that aren't technically necessary to get the desired effect (wait for Vue to finish rendering).

Screenshots (strongly suggested)

N/A just a slight technical change around where to wait for some promises.

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

The testing here still passes as-is and fails if any of the .nextTick() calls in the tests are removed.

This verifies that the change in the source code is correct in intent: Not blocking the calls at the higher level and deferring into the postRender call to await has the same effect without causing any potential blocking at the higher component level.

• 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