Support git rev-list <hash> --count --first-parent over REST API

Without cloning the repository, there should be functionality to get result equivalent to git rev-list <hash> --count --first-parent or git rev-list <hash> --count i.e: the request is to extend existing functionality of GitLab Commit API, to return count of commits for a given SHA.

Proposal:

API End point: /projects/:id/repository/commits/<:sha>/count
Method: GET
Parameters: {--first-parent: boolean} [ optional parameter, default: false]
Response: { "count": <count number> }

Our organisation, make use of this sequence/revision number for a specific workflow and it would be a nice to have this feature integrated in the commit API. Also, we believe this feature request is generic and can be useful for any other organisation as well.