track and report merge request IDs in commit history as a discrete field in the APIs
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### Description
merge commits that are returned from the API should return the merge request ID as a discrete field, instead of being embedded in the message value.
API: `GET /projects/:id/repository/commits/:sha`
Returns an example JSON fragment for merge commits: `"message": "Merge branch 'example' into 'master'\n\nChange the things\n\nSee merge request !123"`
The problem is that I must string parse for the string `\n\nSee merge request !\d+` to get the merge request number. It would be much less error prone if the API returned an optional field for that "merge_request" number
### Proposal
If a commit came from a merge request, in addition to including the existing human readable message, include a field in the JSON response called `merge_request` containing the merge request number.
### Links / references
https://gitlab.com/help/api/commits.md#get-a-single-commit
issue