with: use context manager for wlock in keyword demo

hgext/keyword.py

@@ -444,11 +444,8 b' def demo(ui, repo, *args, **opts):'
     repo[None].add([fn])
     ui.note(_('\nkeywords written to %s:\n') % fn)
     ui.note(keywords)
-    wlock = repo.wlock()
-    try:
+    with repo.wlock():
         repo.dirstate.setbranch('demobranch')
-    finally:
-        wlock.release()
     for name, cmd in ui.configitems('hooks'):
         if name.split('.', 1)[0].find('commit') > -1:
             repo.ui.setconfig('hooks', name, '', 'keyword')