Merge pull request #5362 from nbp/update-channels

Add a script to add git branches for each channel.

To create / update references to remote / local channels, you have to run `./maintainers/scripts/update-channel-branches.sh` while you are at the top-level of nixpkgs work directory.  To make this convenient for Nixpkgs / NixOS developer, one can run the following command:

```
$ git config --add alias.fetch-channels '!sh -c "$(git rev-parse --show-cdup)maintainers/scripts/update-channel-branches.sh"'
```

Which will register the alias fetch-channels such that the script can used from sub-directory of nixpkgs by running `git fetch-channels`.