Matches https://github.com/ilyagr/pijul-clone as of now, might become out of sync

Disallow overwriting existing identities via `pijul id edit`

As pointed out in #736, it is possible to rename an identity to another existing identity. To remedy this the existing name is now passed in as an Option<&String> and checked against user input

Created by  finchie  on November 26, 2022
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