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

locking: remove support for inheriting locks in subprocess This seems to have been added for merge driver, and since merge driver is now gone... Differential Revision: https://phab.mercurial-scm.org/D9053

mergedriver: delete it The merge driver code was added in late 2015. I think '406a654b::6f045b56 & user("sid0")' is a reasonable revset for finding the relevant commits, including preparation for it. The code is very poorly tested, which makes it very hard to maintain. It seems it's only used by FB and they don't use this code base anymore, so let's remove the code to make it easier for us to maintain our product. Differential Revision: https://phab.mercurial-scm.org/D9041