chg: fallback to original hg if stdio fds are missing If stdio fds are missing (ex. fd 0 is not present), chg might open fds that take the numbers 0, and attachio would send the wrong fds to the client, which might cause unwanted behaviors. Avoid that by detecting the missing fds and falling back to the original hg. Differential Revision: https://phab.mercurial-scm.org/D9058