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.

Reproduziere den Fehler, indem du ein Repository mit `git init --object-format=sha256` initialisierst und gitui startest. Beginne damit, die Erkennung des Repositorys und die zugrunde liegenden libgit2-Bindings nachzuverfolgen, die den Fehler „invalid path“ erzeugen; fertig ist die Arbeit, wenn gitui in SHA-256-Repositorys startet und standardmäßige Git-Operationen unterstützt, ohne SHA-1-Repositorys zu beeinträchtigen.

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