How to compare local and remote git files?

Question

I need to list all tracked commited files that only exist locally. Let's say my local repo is a commit forward the remote repo (on github or gitlab) and these are my local tracked commited files:

a.txt
b.txt

Now imagine in my remote repo there is just one of those files:

a.txt

What git command could be used to list the diff between local and remote repos in terms of tracked commited files? To be cristal clear, which git command (git-ls-files, gt-ls-remote, git-ls-tree, etc) could generate the following output:

b.txt

EDIT 1: I have to do this without pulling commits from the remote repo.

EDIT 2: I need this to write a git hook to prevent pushes, but I'm not sure this is a good use case for git hooks.

Answer 1

I think I have figured it out:

git fetch origin && comm -2 -3 \
  <(sort <(git ls-tree -r master --name-only)) \
  <(sort <(git ls-tree -r origin/master --name-only))

Final result:

b.txt

Thank you torek and polareper. Your suggestions helped a lot.

Answer 2

diff -y <(git ls-files) <(git ls-tree -r master --name-only )

I am taking the diff of the files in master to local (ls-files).1