Error when running gren init

When I run gren init, I get the following message:
```
$ gren init
You have to run this command in a git repo folder
```

However, this *is* a git repo folder, i.e. it has a .git subdirectory.

Unfortunately, there is no further information what might be wrong. Any suggestions?

Start by reproducing `gren init` in a Git repository containing a `.git` directory, then trace the repository-detection path used by the command. Done means the command handles a valid repository correctly or reports the actual reason it cannot proceed; the payload names no files or tests to run.

索引モデルが issue の本文から書いたものです。