Follow-up of Forcefully set files status as “unmerged” in a Git repository.

The Copier tool lets you generate projects from templates, and update the generated projects when the template changes.

When updating, it does 3-way merges like this:

git merge-file 
  -L "before updating" 
  -L "last update" 
  -L "after updating" 
  README.md 
  /tmp/project_generated_from_previous_template_version/README.md 
  /tmp/project_generated_from_latest_template_version/README.md

After doing so, the output of git ls-files -s | grep README would look like this:

% git ls-files -s | grep README
100644 8f7a2b8e27bddabb05a3eee508d2b77c33c424f1 0       README.md

README.md’s contents might look like this:

<<<<<<< before updating
Hello girls.
=======
Hello guys.
>>>>>>> after updating

However, whether I pass --ours or --theirs to git checkout, the result is always the “before updating” part (current/ours).

git checkout --ours README.md  # Hello girls. OK.

git checkout --theirs README.md  # Hello girls. I expected Hello guys.

My question is therefore: why? and how to make the checkout command work as I’d expect?

Is this because the files are external to the repository (/tmp/*), and therefore Git doesn’t have the corresponding blobs, and will always use the only blob it knows, which is the current version of the README? I’m not very Git-knowledgeable, so what I’m saying might not make any sense.