Support for SHA-256 object format repositories (fails with "invalid path")

**Is your feature request related to a problem? Please describe.**
I am unable to use gitui in repositories initialized with the SHA-256 object format (git init --object-format=sha256). When I attempt to launch the application in such a repository, it immediately fails and exits with the error message: invalid path: please run gitui inside of a non-bare git repository.

This is frustrating because the repository is completely valid. I can perform all operations using the standard Git CLI without issues. Furthermore, other TUI tools like lazygit handle these SHA-256 repositories correctly, which suggests the issue is specific to gitui or its underlying libgit2 bindings not parsing the repository metadata correctly.

**Describe the solution you'd like**
I would like gitui to detect and support repositories using the SHA-256 hashing algorithm. It should launch normally and allow for standard git operations, just as it does with standard SHA-1 repositories.

**Describe alternatives you've considered**
I have verified that reverting to the default SHA-1 format (git init) allows gitui to work, but this is not a viable solution for my workflow as I specifically require the security properties of SHA-256. Currently, I am forced to use the CLI or alternative tools like lazygit for these specific repositories.

**Additional context**
Here are the steps to reproduce the issue:
1. Initialize a new repository with git init --object-format=sha256.
2. Run gitui.
3. Observe the error: invalid path: please run gitui inside of a non-bare git repository.

`git init --object-format=sha256` でリポジトリを初期化し、gitui を起動して失敗を再現します。まず、リポジトリの検出と、「invalid path」エラーを生成する基盤の libgit2 バインディングを追跡します。gitui が SHA-256 リポジトリで起動し、SHA-1 リポジトリを壊すことなく標準的な Git 操作をサポートできれば完了です。

索引モデルが issue の本文から書いたものです。