autoninja: switch between ninja and siso for help appropriately

When running `autoninja -help` at Chromium project root, it prints
`ninja -help`. `autoninja` should switch between `ninja -help` and `siso
ninja -help` based on the project setting.

Bug: 426149609
Fixed: 426149609
Change-Id: I1a5d1b98e906130ad17061d49a1f25d2c0119c74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6656541
Commit-Queue: Junji Watanabe <jwata@google.com>
Auto-Submit: Junji Watanabe <jwata@google.com>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>

autoninja.py

@@ -387,6 +387,10 @@ def _main_inner(input_args, build_id, should_collect_logs=False):
                 file=sys.stderr,
             )
 
+    # use_siso may not be set when running `autoninja -help` without `-C`.
+    if use_siso is None:
+        use_siso = _get_use_siso_default(output_dir)
+
     siso_marker = os.path.join(output_dir, ".siso_deps")
     if use_siso:
         # siso generates a .ninja_log file so the mere existence of a