help: add a --help-all option to show all commands at once

This is useful when you want to scan all the possibilities of repo
at once.  Like when you're searching for different option names.

Change-Id: I225dfb94d2be78229905b744ecf57eb2829bb52d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/232894
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>