ci: update the git diff command in the prevent deletion workflow to improve accuracy

[Oreoxmt]

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 version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• master (the latest development version)
• v7.3 (TiDB 7.3 versions)
• v7.2 (TiDB 7.2 versions)
• v7.1 (TiDB 7.1 versions)
• v7.0 (TiDB 7.0 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[Oreoxmt]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Oreoxmt

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #14254.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #14255.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.0: #14256.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #14258.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.2: #14259.