(diff list | history)
c = fdtochan(arg, -1, 0, 0);
/* If we used open the chan will be at the first element
* of a union rather than the mhead of the union. undomount
* will make it look like we used Atodir rather than Aopen.
if(c->qid.path & CHDIR)
c = undomount(c);
ptpath(c->path, (char*)arg, arg);