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The idea that the interactive bash prompt isn't set in case of TERM=dumb is intended to fix problems when other machines log remotely into a NixOS installation via Tramp. A side-effect that change was, however, that Emacs' shell-mode no longer had a correct prompt. I suppose the presence of INSIDE_EMACS=24.5.2,comint is a sufficiently unique indication that the current interactive shell is running inside of an Emacs and that the prompt can thus be configured safely.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.