* toggle amend will show changes in HEAD commit
* since discard is not compatible with staged changes in `amend` mode, we only allows user to discard unstaged changes
* remove dotnet-tool.json because the project does not rely on any dotnet tools.
* remove Directory.Build.props because the solution has only one project.
* move src/SourceGit to src. It's not needed to put all sources into a subfolder of src since there's only one project.
