strip: typo bugfix related to '--nobackup -> --no-backup' rename (issue2377)

'--no-backup' in cmdline means *'no_backup'* in code'

hgext/mq.py

@@ -2504,7 +2504,7 b' def strip(ui, repo, *revs, **opts):'
     backup = 'all'
     if opts.get('backup'):
         backup = 'strip'
-    elif opts.get('no-backup') or opts.get('nobackup'):
+    elif opts.get('no_backup') or opts.get('nobackup'):
         backup = 'none'
 
     cl = repo.changelog