autobuilder: Avoid critical if there's no gpg_key
upgrade: Add missing variable initializer
core: Add "rollback" builtin

This is a convenient command to go back to the previous tree.
TODO: Expand a bit
Print treepkgdiff on rollback too

Move it to an internal API.
Put hawkey stuff behind --enable-patched-hawkey-and-libsolv

Right now we depend on two patches.