Add "SHA mismatch" state to the MR widget

What does this MR do?

Adds a new state to the MR widget that is triggered when the most recent poll from the server indicates that the source branch HEAD has changed. This can prevent a reviewer from accidentally merging without realizing that additional changes have been made since he/she reviewed the diff.

Are there points in the code the reviewer needs to double check?

make sure the tests are thorough enough

Why was this MR needed?

See issue #32178 (closed) for a description of the bug implications

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #32178 (closed)