mutt completion doesn't work with neomutt 20200626

[mgedmin]

I've been investigating why mutt completion (neomutt -f =<tab>) doesn't work for me with neomutt 20200626, and I discovered that _muttfiledir() expects neomutt -Q folder to return a value that looks like

folder="/path/to/it"

but neomutt 20200626 returns a value that looks like

set folder = "/path/to/it"

(AFAICT the output format of neomutt -Q changed some time in 2018.)

This is easy to fix by updating the sed regexp, and I'll attempt to prepare a pull request for it.