Unicode renderer: Use vertical ellipsis rather than double-dagger

[joshtriplett]

The Unicode renderer used "‡", the double-dagger symbol, to ellipsize
the vertical bar to indicate elided lines.

Use a vertical ellipsis instead.

[epage]

Thank you for looking to improve this. Unfortunately, this is a "not yet". The priority is matching rustc's output exactly until annotate-snippets is stable. We then need to figure out what is the process for making changes to the output (colors, shapes, etc).

I've been abusing #308 as a place to record thoughts on evolving the output

[joshtriplett]

@epage Fair enough; we should fix this in rustc too, because it's the wrong symbol. :)