It has several advantages:
* reduce the memory costed by histories
* higher performance while parsing commits
* no need to calculate subject every time, which is invoked most frequently to render histories
* 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.
