diff kitten: Do not highlight extremely large files

python's asyncio module has no way to tell a process executor object to
kill its worker processes, so highlighting very large files cause the
kitten to hang around o exit till the highlighting is done

kittens/diff/highlight.py

@@ -122,6 +122,8 @@ def highlight_line(line):
 def highlight_for_diff(path, aliases):
     ans = []
     lines = lines_for_path(path)
+    if len(lines) > 10000:
+        return ans
     hd = highlight_data('\n'.join(lines), path, aliases)
     if hd is not None:
         for line in hd.splitlines():