subcmds: centralize all_commands logic

The branch->branches alias is setup in the main module when that
really belongs in the existing all_commands setup.

For help, rather than monkey patching all_commands to the class,
switch it to use the state directly from the module.  This makes
it a bit more obvious where it's coming from rather than this one
subcommand having a |commands| member added externally to it.

Change-Id: I0200def09bf4774cad8012af0f4ae60ea3089dc0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259153
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>