project: reorganize the structure of the project.

* 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.

src/Models/DiffOption.cs

@@ -0,0 +1,116 @@
+using System.Collections.Generic;
+using System.Text;
+
+namespace SourceGit.Models
+{
+    public class DiffOption
+    {
+        public Change WorkingCopyChange => _workingCopyChange;
+        public bool IsUnstaged => _isUnstaged;
+        public List<string> Revisions => _revisions;
+        public string Path => _path;
+        public string OrgPath => _orgPath;
+
+        /// <summary>
+        ///     Only used for working copy changes
+        /// </summary>
+        /// <param name="change"></param>
+        /// <param name="isUnstaged"></param>
+        public DiffOption(Change change, bool isUnstaged)
+        {
+            _workingCopyChange = change;
+            _isUnstaged = isUnstaged;
+
+            if (isUnstaged)
+            {
+                switch (change.WorkTree)
+                {
+                    case ChangeState.Added:
+                    case ChangeState.Untracked:
+                        _extra = "--no-index";
+                        _path = change.Path;
+                        _orgPath = "/dev/null";
+                        break;
+                    default:
+                        _path = change.Path;
+                        _orgPath = change.OriginalPath;
+                        break;
+                }
+            }
+            else
+            {
+                _extra = "--cached";
+                _path = change.Path;
+                _orgPath = change.OriginalPath;
+            }
+        }
+
+        /// <summary>
+        ///     Only used for commit changes.
+        /// </summary>
+        /// <param name="commit"></param>
+        /// <param name="change"></param>
+        public DiffOption(Commit commit, Change change)
+        {
+            var baseRevision = commit.Parents.Count == 0 ? "4b825dc642cb6eb9a060e54bf8d69288fbee4904" : $"{commit.SHA}^";
+            _revisions.Add(baseRevision);
+            _revisions.Add(commit.SHA);
+            _path = change.Path;
+            _orgPath = change.OriginalPath;
+        }
+
+        /// <summary>
+        ///     Diff with filepath. Used by FileHistories
+        /// </summary>
+        /// <param name="commit"></param>
+        /// <param name="file"></param>
+        public DiffOption(Commit commit, string file)
+        {
+            var baseRevision = commit.Parents.Count == 0 ? "4b825dc642cb6eb9a060e54bf8d69288fbee4904" : $"{commit.SHA}^";
+            _revisions.Add(baseRevision);
+            _revisions.Add(commit.SHA);
+            _path = file;
+        }
+
+        /// <summary>
+        ///     Used to show differences between two revisions.
+        /// </summary>
+        /// <param name="baseRevision"></param>
+        /// <param name="targetRevision"></param>
+        /// <param name="change"></param>
+        public DiffOption(string baseRevision, string targetRevision, Change change)
+        {
+            _revisions.Add(baseRevision);
+            _revisions.Add(targetRevision);
+            _path = change.Path;
+            _orgPath = change.OriginalPath;
+        }
+
+        /// <summary>
+        ///     Converts to diff command arguments.
+        /// </summary>
+        /// <returns></returns>
+        public override string ToString()
+        {
+            var builder = new StringBuilder();
+            if (!string.IsNullOrEmpty(_extra))
+                builder.Append($"{_extra} ");
+            foreach (var r in _revisions)
+                builder.Append($"{r} ");
+
+            builder.Append("-- ");
+            if (!string.IsNullOrEmpty(_orgPath))
+                builder.Append($"\"{_orgPath}\" ");
+            builder.Append($"\"{_path}\"");
+
+            return builder.ToString();
+        }
+
+        private readonly Change _workingCopyChange = null;
+        private readonly bool _isUnstaged = false;
+        private readonly string _orgPath = string.Empty;
+        private readonly string _path = string.Empty;
+        private readonly string _extra = string.Empty;
+        private readonly List<string> _revisions = new List<string>();
+    }
+}