repo: include subcommands in --help output

Also point people to `repo help` so it's easier to navigate the tool.

Bug: https://crbug.com/gerrit/12022
Change-Id: Ib3be331a2cef32caa193640bf8d54bd1443fce60
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/247292


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>