unshelve: create a matcher only if required on creating unshelve ctx

Differential Revision: https://phab.mercurial-scm.org/D6687

mercurial/shelve.py

@@ -836,8 +836,8 b' def _createunshelvectx(ui, repo, shelvec'
     snode = repo.commit(text=shelvectx.description(),
                         extra=shelvectx.extra(),
                         user=shelvectx.user())
-    m = scmutil.matchfiles(repo, repo[snode].files())
     if snode:
+        m = scmutil.matchfiles(repo, repo[snode].files())
         _shelvecreatedcommit(repo, snode, basename, m)
 
     return newnode, bool(snode)