shelve: acquire lock in the right order

Text book says that 'wlock' should be acquired before 'lock'.
Caught through developer warning.

hgext/shelve.py

@@ -544,8 +544,8 b' def unshelve(ui, repo, *shelved, **opts)'
     oldquiet = ui.quiet
     wlock = lock = tr = None
     try:
+        wlock = repo.wlock()
         lock = repo.lock()
-        wlock = repo.wlock()
 
         tr = repo.transaction('unshelve', report=lambda x: None)
         oldtiprev = len(repo)