unbundle: move most of the logic on cmdutil to help debug::unbundle reuse
This make sure `hg debug::unbundle` focus on the core logic.
skip ignored directories if -i or --all not specified
$ hg init t
$ cd t
$ echo 'ignored' > .hgignore
$ hg ci -qA -m init -d'2 0'
$ mkdir ignored
$ ls
ignored
$ hg purge -v --no-confirm
$ ls
ignored