Rename hide_*_incomes function arguments to clarify their effect

[martinholmer]

This pull request implements this suggestion by @MaxGhenis.

[codecov-io]

Codecov Report

Merging #1906 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1906   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        3316    3316           
======================================
  Hits         3316    3316

Impacted Files	Coverage Δ
taxcalc/calculate.py	100% <100%> (ø)	⬆️
taxcalc/utils.py	100% <100%> (ø)	⬆️

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