nixos/installer/channel: Add some files that the channel also has

Nothing probably uses this, but let's be pedantic and have the
pre-included channel on the install media be as close as possible to
what 'nix-channel --update' will give them.

The only remaining difference is that the channel adds programs.sqlite,
which is fundamentally unfixable.

nixos/modules/installer/cd-dvd/channel.nix

···
       if [ ! -e $out/nixos/nixpkgs ]; then
         ln -s . $out/nixos/nixpkgs
       fi
+      echo -n ${config.system.nixos.revision} > $out/nixos/.git-revision
       echo -n ${config.system.nixos.versionSuffix} > $out/nixos/.version-suffix
+      echo ${config.system.nixos.versionSuffix} | sed -e s/pre// > $out/nixos/svn-revision
     '';
 
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