unbundle: move most of the logic on cmdutil to help debug::unbundle reuse
This make sure `hg debug::unbundle` focus on the core logic.
= Mercurial XXX =
== New Features ==
== Default Format Change ==
These changes affect newly created repositories (or new clones) done with
Mercurial XXX.
== New Experimental Features ==
== Bug Fixes ==
== Backwards Compatibility Changes ==
== Internal API Changes ==
== Miscellaneous ==