shelve: split up dounshelve() in unshelvecmd() and _dounshelve()
I'd like to be able to override the new `_dounshelve()`, getting
access to the name of the shelve to unshelve. `unshelvecmd()` seems to
better match the existing `createcmd()`, `listcmd()` etc.
Differential Revision: https://phab.mercurial-scm.org/D8322
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