feat: Add --subtree-only argument and pass it to GitRepo

aider/args.py

@@ -319,6 +319,12 @@ def get_parser(default_config_files, git_root):
         default=default_aiderignore_file,
         help="Specify the aider ignore file (default: .aiderignore in git root)",
     )
+    group.add_argument(
+        "--subtree-only",
+        action="store_true",
+        help="Only consider files in the current subtree of the git repository",
+        default=False,
+    )
     group.add_argument(
         "--auto-commits",
         action=argparse.BooleanOptionalAction,