bug#54843: 29.0.50; rcirc-omit-mode hiding the overlay arrow

[Davide Masserut]

I suspect the overlay arrow inherits the text properties of the
previous character so when the message is omitted by enabling the
invisibility property then the arrow is also hidden. 

Could it be that the problem is in the fact that we allow the arrow to
follow the properties of the surrounding characters?

P.S.
You can also toggle the omitted lines with C-c C-o.