Restore User.from_gitaly

What does this MR do?

Restores Gitlab::Git::User.from_gitaly which was accidentally removed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13264.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The removal of this code breaks the vendoring of lib/gitlab/git into gitaly-ruby.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Broken Gitaly MR: https://gitlab.com/gitlab-org/gitaly/merge_requests/389

Issue to fix TODO introduced in this MR: https://gitlab.com/gitlab-org/gitaly/issues/628