Add spec for plain diff/email patch

What does this MR do?

• Add spec to validate plain diff and email patches options for a commit
• Add supporting page object Page::Project::Commit::Show
• Modify Page::Project::Show to add supporting go_to_commit method
• Add qa-* selectors to commit partial
• Modify qa.rb to load new page object

What are the relevant issue numbers?

#52696 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #52696 (closed)