Setting commit message on composing trees

[Leandro Santiago <leandrosansilva gmail com>]

The command ostree commit has a --body and --subject which allow to
associate a text when commiting changes, but I found no ways of
setting this message on rpm-ostree compose tree or on
rpm-ostre-toolbox treecompose. Such options would be really valuable
on those tools as well.

-- 
Sent from my mind