Fix histplot dodge bar widths with log scale

[mwaskom]

Closes #2819 by adding a test and fixes #2820

[codecov]

Codecov Report

Merging #2849 (3e69307) into master (7b3e4df) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2849   +/-   ##
=======================================
  Coverage   98.04%   98.04%           
=======================================
  Files          69       69           
  Lines       22242    22258   +16     
=======================================
+ Hits        21807    21823   +16     
  Misses        435      435           

Impacted Files	Coverage Δ
seaborn/distributions.py	96.31% <100.00%> (+0.03%)	⬆️
tests/test_distributions.py	99.79% <100.00%> (+<0.01%)	⬆️

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