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/Commands/Config.cs

@@ -0,0 +1,80 @@
+using System;
+using System.Collections.Generic;
+
+namespace SourceGit.Commands
+{
+    public class Config : Command
+    {
+        public Config(string repository)
+        {
+            WorkingDirectory = repository;
+            Context = repository;
+            RaiseError = false;
+        }
+
+        public Dictionary<string, string> ListAll()
+        {
+            Args = "config -l";
+
+            var output = ReadToEnd();
+            var rs = new Dictionary<string, string>();
+            if (output.IsSuccess)
+            {
+                var lines = output.StdOut.Split(new char[] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries);
+                foreach (var line in lines)
+                {
+                    var idx = line.IndexOf('=', StringComparison.Ordinal);
+                    if (idx != -1)
+                    {
+                        var key = line.Substring(0, idx).Trim();
+                        var val = line.Substring(idx + 1).Trim();
+                        if (rs.ContainsKey(key))
+                        {
+                            rs[key] = val;
+                        }
+                        else
+                        {
+                            rs.Add(key, val);
+                        }
+                    }
+                }
+            }
+
+            return rs;
+        }
+
+        public string Get(string key)
+        {
+            Args = $"config {key}";
+            return ReadToEnd().StdOut.Trim();
+        }
+
+        public bool Set(string key, string value, bool allowEmpty = false)
+        {
+            if (!allowEmpty && string.IsNullOrWhiteSpace(value))
+            {
+                if (string.IsNullOrEmpty(WorkingDirectory))
+                {
+                    Args = $"config --global --unset {key}";
+                }
+                else
+                {
+                    Args = $"config --unset {key}";
+                }
+            }
+            else
+            {
+                if (string.IsNullOrWhiteSpace(WorkingDirectory))
+                {
+                    Args = $"config --global {key} \"{value}\"";
+                }
+                else
+                {
+                    Args = $"config {key} \"{value}\"";
+                }
+            }
+
+            return Exec();
+        }
+    }
+}