refactor: use $GIT_COMMON_DIR instead of cut $GIT_DIR/worktrees (#1103)

src/Commands/QueryGitCommonDir.cs

@@ -0,0 +1,26 @@
+using System.IO;
+
+namespace SourceGit.Commands
+{
+    public class QueryGitCommonDir : Command
+    {
+        public QueryGitCommonDir(string workDir)
+        {
+            WorkingDirectory = workDir;
+            Args = "rev-parse --git-common-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));
+        }
+    }
+}