Homebrew update failed: Runtime error: git pull -q origin refs / heads / master: refs / remotes / origin / master

When I try to run brew update

I see:, error: The following untracked working tree files would be overwritten by merge:

then a large list of packages, and then I get an error:

Error: Failure while executing: git pull -q origin refs/heads/master:refs/remotes/origin/master

...

What can I do about it?

Thank!