Add support for "unshelve --keep"
Created originally on Bitbucket by wil_ (wil)
hg unshelve has the --keep option to unshelve changes into the working directory, without losing the contents of the shelve: https://www.selenic.com/mercurial/hg.1.html#shelve
TortoiseHg's implementation of the shelve/unshelve feature does not seem to have this functionality. It would be very useful to add it.