Option for not removing shelve after `unshelve'

Sometimes it is useful to not remove the shelve after `bzr unshelve'.

git supports this with `git stash apply'.

Say you have something shelved.

You want to try to refine the change, unshelve it and continue working
on it. If you realize that what you had before was better, there's no easy way to go back.

Or if you want to split a change:
shelve
apply
remove the part you don't want
shelve the partial change
unshelve the original
remove some other part
shelve again

I suggest `bzr unshelve --keep' or `bzr unshelve --apply'.