* do NOT modify the existing merge, and add a new constructor for `Commands.Merge` instead
* rewrite `ViewModels.MergeMultiple`
- since `_histories.Commits.Find` may returns null, use `List<object>` instead of `List<Models.Commits>`
- supports display merge target as both `Models.Commit` and `Models.Branch`
* rename translation key `Text.MergeMultiple.Commit` to `Text.MergeMultiple.Targets`, and add translations for zh_CN and zh_TW
* some UI/UX 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.
