This probably doesn't affect the author and commit stats for mozilla-central all that much. The various upstream libraries we use are updated infrequently, in single commits.
The libraries under github.com/servo are just Servo components (some of them are used by other people, though) which are in different repos. These are maintained by the Servo team and in many cases Servo-specific, for example our HTML parser, CSS parser, URL library, and our layers/geometry logic. Also, eventually, most of our rendering logic.