color: rename '_styles' to '_defaultstyles' for clarity
This should make it clear the dict is only used for new config. Extensions
should not modify it directly anyway since we have 'extraloader' logic for
loading '_styles' too.
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
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273ce12ad8f1` will report some style violations on a very old commit).