I use the API function get single Merge request.
Is it possible to find it out the commit sha of the last commit without using Get single MR commits Just with the get single merge request call?
I think this is what you are looking for,
Get a single commit
Get a specific commit identified by the commit hash or name of a branch or tag.
GET /projects/:id/repository/commits/:sha
From the GitLab wesite,
https://docs.gitlab.com/ee/api/commits.html#get-a-single-commit