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.