git init
git remote add origin url://to/source/repository
git fetch origin # SHA #
git reset --hard FETCH_HEAD
git init
git remote add origin url://to/source/repository
git fetch origin # SHA #
git reset --hard FETCH_HEAD
GitHub supports fetching arbitrary SHAs if e.g. a branch was force-pushed and you want to fetch the state before the forced push. Maybe this case counts as "advertised object SHA".
The commit hash must not be abbreviated.
Man, you saved me, thanks