Add support for SHA-256-hashed repositories

Hi!

Does by any chance this plugin fail to work with SHA256 repositories?

```
ERROR: core/variant/variant_utility.cpp:1098 - GitPlugin: Could not initialize repository. Error -1: unsupported extension name extensions.objectformat in godot-git-plugin\src\git_plugin.cpp:GitPlugin::_initialize#L695
ERROR: Could not initialize GitPlugin
```

This is the error I get when trying to turn on VSC for my project. Note that my project was initialized with default Git source control metadata; I then deleted the `.git` folder to initialize a new repository using `git init --object-format=sha256`.

I have tried to turn on VSC for an older project that uses default SHA1 and it works as expected.