feat: extract and test pre-commit-autoupdate diff parsing

[blink1073]

Summary

• Extracts the inline diff-parsing loop from pre-commit-autoupdate/action.yml into a standalone parse_diff.sh script
• Strips single and double quotes from captured version strings
• Adds test_parse_diff.sh covering unquoted, single-quoted, and double-quoted rev values
• Adds test_pre_commit_config.yaml sample config with all three quote styles
• Wires Test parse_diff.sh into the test_scripts job in tests.yml