repoguard: allow repo parameter formats like /handle/repo
closes #42 by stripping one leading slash from the repo parameter.
also fixes a minor issue with the nix develop shellhook.
this kinda hacky fix is tested with the following commands:
```
nix build .#repoguard
SSH_ORIGINAL_COMMAND="git-upload-pack '/soopy.moe/test-repo'" ./result/bin/repoguard -log-path /tmp/ASDUJBIf-repoguard.log -user git
SSH_ORIGINAL_COMMAND="git-upload-pack 'soopy.moe/test-repo'" ./result/bin/repoguard -log-path /tmp/ASDUJBIf-repoguard.log -user git
```