Add "rpm" command, for doing rpm/yum queries on commits.

 It currently has the following sub-commands:

diff COMMIT COMMIT
 for rpmtree diff.
list [prefix...] COMMIT...
 for "yum list" like command.
version COMMIT...
 for "yum version" like command.

...bunch of FIXME's, UI output isn't great, needs docs.
 We also don't use the same code as the treediff on upgrade atm.
Namespace and share DEFINE_TRIVIAL_CLEANUP_FUNC via. rpmostree-util.h
Add two column output for rpm list/diff, to look more like yum list.
 Move from using user commit name, to found commit checksum.

 We don't want dirs. for branch names etc. so always use the checksum.