git bundle create <remote_name>..<local_reference_name>
Eg.
git bundle create origin..i40-faster-bundle-creation
This doesn't work if repo contains a branch named "origin".
Alternative:
git rev-parse $(git log HEAD --not --remotes=origin --pretty=format:%H | tail -n 1)^
"Find all not pushed commits in 'origin' starting from HEAD, take the last one and than find the parent".