autoninja: Show additional help information for --help, like for -h.

This doesn't catch all the variants of --help that getopt allows but it
does catch --help, which is probably the most common.

Bug: 329317224
Change-Id: I2dde63df84bd2c2ade5f9d2c1ab10c3dc13fba70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5370829
Auto-Submit: David Baron <dbaron@chromium.org>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>

autoninja.py

@@ -218,7 +218,7 @@ def main(args):
             output_dir = arg[2:]
         elif arg in ("-o", "--offline"):
             offline = True
-        elif arg == "-h":
+        elif arg in ("-h", "--help"):
             print(
                 "autoninja: Use -o/--offline to temporary disable remote execution.",
                 file=sys.stderr,