[meld] treehelpers: Fix previous-treepath helper

[Kai Willadsen <kaiw src gnome org>]

commit 5fe6a0849228f1f2e48bfd7924f4ab76237869f0
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Sun Dec 8 06:19:04 2019 +1000

    treehelpers: Fix previous-treepath helper

 meld/treehelpers.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/meld/treehelpers.py b/meld/treehelpers.py
index abf0787b..16c0cb84 100644
--- a/meld/treehelpers.py
+++ b/meld/treehelpers.py
@@ -29,7 +29,7 @@ def tree_path_as_tuple(path):
 def tree_path_prev(path):
     if not path or path[-1] == 0:
         return None
-    return path[:-1] + (path[-1] - 1,)
+    return path[:-1] + [path[-1] - 1]
 
 
 def tree_path_up(path):