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
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a