Test fix reading group.

- Gitolite.AddUserGroup() is in fact AddUserOrRepoGroup():
it shouldn't mark the group as a user group
- Gitolite.addUsersGroup(grp *Group) should only update userGroups,
not groups.
Test Fix Gitolite.addUsersGroup()

Adds group in Gitolite.groups as well as userGroups

Uses gtl.AddUserOrRepoGroup() instead of old name gtl.AddUserGroup()
Add grp.markAsUserGroup() where it is needed.
Test reader fix gtl.namesToGroups management

Add name to namesToGroups when addReposGroup() or AddUserOrRepoGroup()
Avoid duplication for namesToGroups
Avoid duplication for addGroup()