Bug#866366: mutt: my index_format setting is corrupted with newer (neo)mutts

[Jonathan Dowland]

Just a gentle reminder about this, I hit it again when my mutt updated
with the recent binnmu so I lost my local fix. The patch in #18 works
for me, and I think has been applied upstream.

   https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=866366#18