nix: Move shell hook from default.nix -> shell.nix and set custom shell prompt.

default.nix

@@ -93,21 +93,6 b' let'
         pythonPackages = self;
       };
 
-      # Somewhat snappier setup of the development environment
-      # TODO: move into shell.nix
-      # TODO: think of supporting a stable path again, so that multiple shells
-      #       can share it.
-      shellHook = ''
-        # Set locale
-        export LC_ALL="en_US.UTF-8"
-
-        tmp_path=$(mktemp -d)
-        export PATH="$tmp_path/bin:$PATH"
-        export PYTHONPATH="$tmp_path/${self.python.sitePackages}:$PYTHONPATH"
-        mkdir -p $tmp_path/${self.python.sitePackages}
-        python setup.py develop --prefix $tmp_path --allow-hosts ""
-      '';
-
       # Add VCSServer bin directory to path so that tests can find 'vcsserver'.
       preCheck = ''
         export PATH="$out/bin:$PATH"

shell.nix

@@ -9,10 +9,28 b' let'
       pkgs;
   };
 
+  vcs-pythonPackages = vcsserver.pythonPackages;
+
 in vcsserver.override (attrs: {
 
   # Avoid that we dump any sources into the store when entering the shell and
   # make development a little bit more convenient.
   src = null;
 
+  # Somewhat snappier setup of the development environment
+  # TODO: think of supporting a stable path again, so that multiple shells
+  #       can share it.
+  postShellHook = ''
+    # Set locale
+    export LC_ALL="en_US.UTF-8"
+
+    # Custom prompt to distinguish from other dev envs.
+    export PS1="\n\[\033[1;32m\][VCS-shell:\w]$\[\033[0m\] "
+
+    tmp_path=$(mktemp -d)
+    export PATH="$tmp_path/bin:$PATH"
+    export PYTHONPATH="$tmp_path/${vcs-pythonPackages.python.sitePackages}:$PYTHONPATH"
+    mkdir -p $tmp_path/${vcs-pythonPackages.python.sitePackages}
+    python setup.py develop --prefix $tmp_path --allow-hosts ""
+  '';
 })