at the moment you cannot get a consolidated list of repository permissions for a user. To a part this is due to the fact, that SCM-Manager cannot always determine the groups a user is member of (for example when you use an external authentication like LDAP), unless she is logged in.
Using /v2/users/<userid>/permissions, you will always get the global permissions set for the user. Repository specific permissions for groups or users are always available under /v2/repositories/<namespace>/<name>/permissions/.
Nonetheless we have this feature on our to-do list, so keep your eyes open.