Fix error when undoing too far

Remove undo history before story list is written to buffer so it is no
longer possible to undo all the way back to an empty buffer which would
cause an error in the Python save_pos() and recall_pos() functions.

Author: ryanss

ftplugin/hackernews.py

@@ -72,6 +72,9 @@ def main():
     vim.command("setlocal noswapfile")
     vim.command("setlocal buftype=nofile")
 
+    if vim.eval("changenr()") == "1":
+        vim.command("setlocal undolevels=-1")
+
     bwrite("┌───┐")
     bwrite("│ Y │ Hacker News (news.ycombinator.com)")
     bwrite("└───┘")
@@ -116,6 +119,7 @@ def main():
             line %= (" "*4, item['time_ago'], item['id'])
             bwrite(line)
         bwrite("")
+    vim.command("setlocal undolevels=100")
 
 
 def link(external=False):