Double negatives

[glacials]

Open this run: https://splits.io/861p

And hover over the second-to-last split in the timeline:

Looks like format_casual may have never worked for negatives and we just never noticed, because we tend to use pretty_difference for negative use cases.

This is two issues:

• When format_casual displays a negative number, it shows two - characters instead of one
• The possible timesave for this segment is negative which doesn't make sense. If it's us then this is a bug to be fixed, if this is a corrupted split file then we should at least hide the number instead of rendering one that doesn't make sense. Hopefully also find out how a file could get corrupted like this through normal use.