this is how it used to be:
git update-index --assume-unchanged <file>
git update-index --no-assume-unchanged <file>
but later on:git update-index --skip-worktree …
"assume" is said to be for large files so Git won't check them for changes. Also, on theirs every change in upstream they will be tracked again (which might be good but not always).
git symbolic-ref HEAD refs/heads/otherbranch