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/QueryGitDir.cs

@@ -0,0 +1,26 @@
+using System.IO;
+
+namespace SourceGit.Commands
+{
+    public class QueryGitDir : Command
+    {
+        public QueryGitDir(string workDir)
+        {
+            WorkingDirectory = workDir;
+            Args = "rev-parse --git-dir";
+            RaiseError = false;
+        }
+
+        public string Result()
+        {
+            var rs = ReadToEnd().StdOut;
+            if (string.IsNullOrEmpty(rs))
+                return null;
+
+            rs = rs.Trim();
+            if (Path.IsPathRooted(rs))
+                return rs;
+            return Path.GetFullPath(Path.Combine(WorkingDirectory, rs));
+        }
+    }
+}