abandon_stale.py: fix typo in option handling

Change-Id: I8950cd26fa852a394b1984e52b0f74092d6f8db5
diff --git a/contrib/abandon_stale.py b/contrib/abandon_stale.py
index 958077d..5f5b9ef 100755
--- a/contrib/abandon_stale.py
+++ b/contrib/abandon_stale.py
@@ -135,7 +135,7 @@
         query_terms = ["status:new", "age:%s" % options.age]
         if options.branches:
             query_terms += ["branch:%s" % b for b in options.branches]
-        elif options.exclude_branch:
+        elif options.exclude_branches:
             query_terms += ["-branch:%s" % b for b in options.exclude_branches]
         if options.projects:
             query_terms += ["project:%s" % p for p in options.projects]