https://developer.atlassian.com/static/rest/stash/3.10.2/stash-rest.html#idp2008384
/rest/api/1.0/projects/{projectKey}/repos/{repositorySlug}/commits/{commitId}/diff/{path:.*}
Where does the 2nd commit ID go? There's supposed to be a source and destination ID but it's unclear how to format this, and there are no example queries.