WIP: Github Actions: Fix "Get changed files" in Shellcheck workflow

[andygrunwald]

✍️ Description

The Shellcheck GitHub Action workflow is not running. It throws an error in the "Get changed files" step:

Run if true; then
fatal: ambiguous argument 'HEAD^1': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

See https://github.com/community-scripts/ProxmoxVE/actions/runs/12724031265/job/35469760341

This Pull Request is using the same "Get changes files" logic from another workflow:

ProxmoxVE/.github/workflows/validate-filenames.yml

Lines 36 to 43 in cc40b59

	- name: Get changed files
	id: changed-files
	run: |
	if ${{ github.event_name == 'pull_request_target' }}; then
	echo "files=$(git diff --name-only ${{ github.event.pull_request.base.sha }} ${{ steps.pr.outputs.result && fromJSON(steps.pr.outputs.result).merge_commit_sha }} | xargs)" >> $GITHUB_OUTPUT
	else
	echo "files=$(git diff --name-only ${{ github.event.before }} ${{ github.event.after }} | xargs)" >> $GITHUB_OUTPUT
	fi

Work in Progress!

This Pull Request is work in progress as the testing can only be done via Github actions.
I will comment, once this is ready.

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🛠️ Type of Change

• Bug fix (non-breaking change that resolves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change unexpectedly)
• New script (a fully functional and thoroughly tested script or set of scripts)

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✅ Prerequisites

• Self-review performed (I have reviewed my code to ensure it follows established patterns and conventions.)
• Testing performed (I have thoroughly tested my changes and verified expected functionality.)
• Documentation updated (I have updated any relevant documentation)

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📋 Additional Information (optional)

An alternative would be to use https://github.com/marketplace/actions/changed-files instead of custom crafted shell scripts.

[andygrunwald]

Superseeded by #1423