Use reverse video not colors for topic change when disabled

`-H 0,0`/"hash = 0,0" makes catgirl mostly colorless which is great,
but topic changes still hardcode brown/green colors to show differences
which is usually not desired by users (like me) disabling colors.

Go for a less eye stressing topic change message that shows both old
and new in reverse video with default terminal colors.

This isn't perfect, other parts of catgirl still hardcode colors and
`-H 0,0`/"hash = 0,0" was never meant to disable colors completely, but
topics change often enough that avoiding less readable^Waccessible topic
diffs seems sensible enough.

NB: parseHash() is brittle and "0,0" is not the only value disabling
colors...

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