Use git worktree in diffcheck.sh to avoid mutating the working tree

[viktorking7]

Summary

Replace git checkout HEAD^ with a temporary detached git worktree in scripts/diffcheck.sh. This eliminates side effects on the current branch and ensures clean cleanup.

Changes

• Add strict shell mode: set -Eeuo pipefail
• Use mktemp for temp files/dirs
• Build previous commit table in a detached worktree
• Add trap cleanup to remove worktree and temps

Why

• Prevents leaving the repo in detached HEAD
• Avoids accidental loss of uncommitted changes
• More CI-friendly and reproducible

[jonatack]

Also, note that this change did not pass CI. Putting this into draft until it does.

[jonatack]

@viktorking7, this is still not passing CI. There is a similar, already existing proposal in #1924 -- would you mind reviewing it, please?