streaming git blame: add users fields

[jhchabran]

We need some user fields in the streamed git blame responses:

https://github.com/sourcegraph/sourcegraph/pull/44287#discussion_r1064535584

@philipp-spiess wrote:

Yeah this is still required. With the GraphQL query it's possible to load the user from a git author but with our streaming endpoint this is not included yet. Do you think it would be a lot of work to add this to the JSON payloads?