fix gm clone of empty repo

[amtoine]

the error before

warning: You appear to have cloned an empty repository.
fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
Error: nu::shell::access_beyond_end

  × Row number too large (empty content).
    ╭─[/home/amtoine/.local/share/nupm/modules/nu-git-manager/git/repo.nu:24:44]
 23 │     let repo = $repo | default (pwd)
 24 │     (^git -C $repo rev-list HEAD | lines | last)
    ·                                            ──┬─
    ·                                              ╰── index too large (empty content)
 25 │ }
    ╰────

the cloning now

warning: You appear to have cloned an empty repository.
it appears you have cloned an empty repository
creating an 'init' commit
[main (root-commit) 8803d51] init
updating cache... done

[amtoine]

same as in #200, i'll merge this to move on and because the "doc" pipeline is green.