E2E test: use commit_email for checking diffs

What does this MR do and why?

With !150855 (merged), in the qa/qa/specs/features/browser_ui/3_create/repository/user_views_commit_diff_patch_spec.rb test we no longer send in explicit author_email when creating a commit via the API, this means that these commits will have the commit_email of the user who created the commit by default. This change was made because it is confusing to use a different email than the user's email (even though you can).

This has caused a failure issue in production https://gitlab.com/gitlab-org/gitlab/-/issues/458615 where the test was previously explicitly setting the email address of the commit to the public_email of the user (in prod the gitlab qa doesn't have a public email, so it defaults to the default_email in the tests which is 'admin@example.com'). So when validating this fails as the commit now uses the commit_email of the user which is gitlab-qa@gitlab.com.

Now in the test we will use the commit_email of the user by default, so when validating the commit we need to use the commit_email also: !150855 (diffs)

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/458615

This test doesn't fail in staging as the gitlab qa user in staging has the same public_email as their commit_email

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How to set up and validate locally

QA_LOG_LEVEL=DEBUG QA_GITLAB_URL=https://gdk.test:3443 bundle exec rspec qa/specs/features/browser_ui/3_create/merge_request/view_merge_request_diff_patch_spec.rb

I've also tested this test against staging.