Fold a git-subtree push script into a rebuild script safely

In a NixOS flake repo with a git subtree that has its own sync script (commit subtree path only, then git subtree split and push to a separate remote), calling that script from the rebuild script has two gotchas. First, if the rebuild script excludes the subtree from staging and exits early on git diff --cached --quiet, a subtree-only change never triggers a rebuild, even when that subtree holds Nix inputs like plugins.nix. Record git rev-parse HEAD before the sync and skip the early exit when HEAD moved. Second, the later generation commit then has nothing staged and git commit fails under set -e, so guard it with git diff --cached --quiet || git commit. Call the sync as sync.sh || echo warn so a subtree remote conflict does not abort the whole rebuild.