show: Simplify new text formatter code

show: Simplify new text formatter code

This makes the text formatter take advantage of the new code
structure.  The previously duplicated header logic is now unified,
several things that we used to compute repeatedly across different
callbacks are now computed once, and the code is simpler overall and
32% shorter.

Unifying the header logic causes this to format some dates slightly
differently, so the two affected test cases are updated.