Tweaks to changelog generator script. (#11280)
Tweaks to changelog generator script. (#11280)

+ Adds a Performance category.
+ Have the examples of the --new option be
  different from those of the --prior option,
  because that's how they wil be used in practice.
+ Clarifies what to do with internal-only commits.

[ci skip-rust]
pantsbuild/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,jsirois/pants,jsirois/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,jsirois/pants,benjyw/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants