ci: update the git diff command in the prevent deletion workflow to improve accuracy (#2417)

[ti-chi-bot]

This is an automated cherry-pick of #2417

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What is changed, added, or deleted? (Required)

Fix an issue that the prevent-deletion.yaml workflow mistakenly detects image deletions when images are added or removed in the PR's base branch. The root cause of this issue is that the workflow directly compares the latest commit of the PR base branch with the latest commit of the PR.

This PR improves the accuracy of the image deletion detection script by adding the --merge-base option to the git diff command. This change ensures that git calculates the changes from the shared starting point of the HEAD and the PR head SHA.

Which TiDB Operator version(s) do your changes apply to? (Required)

• master (the latest development version)
• v1.5 (TiDB Operator 1.5 versions)
• v1.4 (TiDB Operator 1.4 versions)
• v1.3 (TiDB Operator 1.3 versions)
• v1.2 (TiDB Operator 1.2 versions)

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[Oreoxmt]

/approve

[ti-chi-bot]

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