softstrip: fix a reference to an undefined variable

`backupfile` wasn't defined if no backup was requested. Let's set it
to `None` by default, which matches what regular `repair.strip()`
does.

Differential Revision: https://phab.mercurial-scm.org/D9985

mercurial/repair.py

@@ -308,6 +308,7 b' def softstrip(ui, repo, nodelist, backup'
     if not tostrip:
         return None
 
+    backupfile = None
     if backup:
         node = tostrip[0]
         backupfile = _createstripbackup(repo, tostrip, node, topic)