✨ plugins: add `$GIT_DIRECTORY_NAME` env variable

[a-kenji]

Expose the $GIT_DIRECTORY_NAME environment variable to the git_repositories plugin.

This is useful for setting app_ids, or tmux session names, for example.

I am not sure about error handling here. The unwraps are there because:

• We have a git directory, the file name should always succeed.
• It should always be valid Utf8.

We could return an empty string in an error case, that way scripts might still succeed and fill in a default value.