Under GNU Screen, the directional marker causes display corruption in
the index.

This (along with past filters added) should perhaps be considered GNU
Screen bugs.  They've been reported upstream a while ago, but so far
not received any attention.  So for Mutt users' benefit it's better to
filter them out for now.

Thanks to Vincent Lefèvre for debugging and reporting the problem,
along with providing historical information from similar past issues.