* tooltip for swap button
* move `Views.CompareTargetWorktree` to `Models.CompareTargetWorktree`
* remove unused `Models.IObjectId`
* fix swap not working when target is Worktree, because Commands.CompareRevisions's Args do not changed after swapping
* this tooltip does NOT update until it's owner row recreated or the DataContext of that row changed. You can scroll it out of bounds to force refresh the tooltip
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.
