channel.nix: Use filterSource to exclude unnecessary files from nixpkgs

Currently, if you try to build a NixOS config including channel.nix, e.g.:

nix-build -I nixpkgs=. -I nixos-config=nixos/modules/installer/cd-dvd/installation-cd-minimal.nix nixos -A config.system.build.isoImage

twice in a row, you end up with two different build results. This is
caused by the 'result' symlink of the first build affecting the channel
contents of the second build.

If we use filterSource with a predicate that ignores the 'result'
symlinks, the problem is gone. Do the same thing for VIM/Emacs
swap/backup files to avoid even more 'spurious' rebuilds.

Additionally, filter out the '.git' directory at the same time, as we
'rm -rf' it from the result anyway. This avoids a considerable amount of
unnecessary file I/O copying and deleting the .git directory.