unbundle: move most of the logic on cmdutil to help debug::unbundle reuse
This make sure `hg debug::unbundle` focus on the core logic.
$ hg init a
$ cd a
$ touch a; hg commit -qAm_
$ hg bookmark $(for i in $($TESTDIR/seq.py 0 20); do echo b$i; done)
$ hg clone . ../b -q
$ cd ../b
Checking that when lookup multiple bookmarks in one go, if one of them
fails (thus causing the sshpeer to be stopped), the errors from the
further lookups don't result in tracebacks.
$ hg pull -r b0 -r nosuchbookmark $(for i in $($TESTDIR/seq.py 1 20); do echo -r b$i; done) ssh://user@dummy/$(pwd)/../a
pulling from ssh://user@dummy/$TESTTMP/b/../a
abort: unknown revision 'nosuchbookmark'
[255]