npins add git breaks if both --at <rev> and --branch are given

[stuebinm]

This should probably just raise an error when doing npins add git; either one uses it with --at <rev> --branch <name> or with --at <tag>. But currently, giving both appears to work, until it doesn't:

❯ npins init --bare
[INFO ] Welcome to npins!
[INFO ] Creating `npins` directory
[INFO ] Writing default.nix
[INFO ] Writing initial sources.json (empty)
[INFO ] Successfully written initial files to 'npins'.

❯ npins add git https://stuebinm.eu/git/nixq --at 0.0.2 --branch main
[INFO ] Adding 'nixq' …
    repository: https://stuebinm.eu/git/nixq
    branch: main
    revision: 0.0.2
    hash: 187p0i8dxy4glk8g1hwfvm8vl4pgfv9qj3lmy4rh8vjvygcqx8p1

❯ nix-instantiate --eval npins -A nixq.outPath
error:
       … while calling the 'fetchGit' builtin
         at /tmp/flake/npins/default.nix:26:43:
           25|       })
           26|     else assert repository.type == "Git"; builtins.fetchGit {
             |                                           ^
           27|       url = repository.url;

       error: hash '0.0.2' has wrong length for hash type 'sha1'