Fix run on Github
It failed with the following error message:
++ git diff --name-only origin/main...origin/jjasghar/docs.instruct
fatal: ambiguous argument 'origin/main...origin/jjasghar/docs.instruct': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
After testing manually (eg, cut and paste the commands from a failing run), it seems that the git repo is checkout at the exact ref of the PR, so we do not need to use $GITHUB_HEAD_REF (who do not seems to resolve to a commit by default, despites what some examples on the web say).
'...' was removed since that's equivalent to '..' for git-diff (see man pages)