Currently, `git cl help baseurl` fails as it tries to execute the
subcommand, completely bypassing the help.

Parse the arguments early so that the help command runs cleanly for
baseurl.

Fixed: 1345909
Change-Id: Icad7aca6cdc7c9938e91a421cb72e08433a30fd4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3776942
Auto-Submit: Thiago Perrotta <tperrotta@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>