API feature request: List project merge requests by source branch

Using the API, I would like to be able to list project merge requests by source branch.

This could be done by extending List Project Merge Requests with a source_branch attribute.

https://docs.gitlab.com/ee/api/merge_requests.html#list-project-merge-requests

GET /projects/:id/merge_requests?source_branch=my_branch_name

This would be useful for building some tools (for example, Gitlab CE's own bin/changelog script could use it to autocomplete the MR number if one has already been created)