Factor out common percentage formatting code.

[aalexand]

No description provided.

[codecov-io]

Codecov Report

Merging #265 into master will decrease coverage by 0.15%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
- Coverage    65.2%   65.05%   -0.16%     
==========================================
  Files          34       34              
  Lines        7174     7163      -11     
==========================================
- Hits         4678     4660      -18     
- Misses       2102     2109       +7     
  Partials      394      394

Impacted Files	Coverage Δ
internal/measurement/measurement.go	33.64% <0%> (-1.86%)	⬇️
internal/report/source.go	80.77% <100%> (ø)	⬆️
internal/graph/dotgraph.go	91.21% <100%> (+1.21%)	⬆️
internal/report/report.go	30.41% <16.66%> (-1.09%)	⬇️

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