color: move '_colormode' to the core 'ui' class
Having all 'ui' objects aware of 'color' allows us to update the core code to
handle color. The mode will stay 'None' in the default case so that will not
introduce any changes.
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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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commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).