shelve: execute checkunfinished inside wlock scope

Before this patch, "hg shelve" of shelve extension executes
'cmdutil.checkunfinished()' before acquisition of wlock.

It may cause unintentional result, if another command runs parallelly
(see also issue4368).

To avoid this issue, this patch executes 'cmdutil.checkunfinished()'
inside wlock scope of "hg shelve".

This also fixes issue4957, because now 'cmdutil.checkunfinished()'
isn't invoked at "hg shelve" with options below:

  --cleanup
  --delete
  --list
  --patch
  --stat

hgext/shelve.py

@@ -226,6 +226,7 b' def createcmd(ui, repo, pats, opts):'
     """subcommand that creates a new shelve"""
     wlock = repo.wlock()
     try:
+        cmdutil.checkunfinished(repo)
         return _docreatecmd(ui, repo, pats, opts)
     finally:
         lockmod.release(wlock)
@@ -808,8 +809,6 b' def shelvecmd(ui, repo, *pats, **opts):'
     To delete specific shelved changes, use ``--delete``. To delete
     all shelved changes, use ``--cleanup``.
     '''
-    cmdutil.checkunfinished(repo)
-
     allowables = [
         ('addremove', set(['create'])), # 'create' is pseudo action
         ('cleanup', set(['cleanup'])),