Rebuild-skip check: user wanted git-based, not nix eval

When asked to make a NixOS rebuild script skip rebuilding when there are no changes, the user meant git changes, not a system-closure comparison. Comparing nix eval of the toplevel outPath to /run/current-system costs ~30s of silent eval before every run and read as a hang. Cheaper and what was wanted: record git rev-parse HEAD before git pull, then after nix flake update and git add -A, skip if HEAD is unchanged and git diff --cached --quiet. Also: a skipped rebuild must not commit; only commit after an actual switch, matching the script's original contract.