Allow retrieving commits for merge request

## Description of the problem, including code/CLI snippet
API does not support getting commits for merge request (or documentation hasn't been updated)

see: https://docs.gitlab.com/ce/api/merge_requests.html#get-single-mr-commits

## Expected Behavior
`project.commits()` should return a list of commits

## Actual Behavior
Not defined in docs

## Specifications
v4 (and v3?)
