This is the last issue to complete the port to Golang (#2). Once we know that it's a `hg` command and we've got the internal REST API response, we need to `exec` the inner `hg serve -R <REPO_PATH> --stdio` command with the proper environment variables. Most of these environment variables are mandatory, notably `REMOTE_USER` (expresses permission), and `HGRCPATH`. Some are less important, such as `HEPTAPOD_PROJECT_PATH` (see heptapod#72). All of them are easy to fill: that's what the API Response is there for. In the Ruby version this is done in [exec_hg_cmd](lib/gitlab_shell.rb#L259), where the full list of needed environment variables can be seen.