Create a second Unity project, and edit the manifes.json as follows:
{
"dependencies": {
"com.newhere_tools.git_un": "file:C:/Users/you/where_you_stored_it/unity_git_collaboration_plugin/Assets/git_Un/",
...
}
}
The file path is the path to this repository, here you make the changes to the code, but testing it happens in the second Unity project, which ensures, it is tested in the setting it will be used: as an imported package.