[feat] avoid unchanged revisions

[moritztim]

using -u, --update [ URL | ID ] you can create a revision to agist without any changes.

$ gist unchanged_file.txt
https://gist.github.com/moritz-t-w/4516587d63e1d8d9d10d7c111585cab8
$ gist -u 4516587d63e1d8d9d10d7c111585cab8 unchanged_file.txt
https://gist.github.com/moritz-t-w/4516587d63e1d8d9d10d7c111585cab8