phase-shelve: also capture the state of shelve prior to unshelve

tests/test-shelve.t

@@ -1594,4 +1594,8 b' shelve --list --patch should work even w'
   $ hg add somefile.py
   $ hg shelve -q
   $ hg log --hidden
+  $ ls .hg/shelved
+  default.hg
+  default.patch
+  default.shelve
   $ hg unshelve -q