color: set initial default value for 'colormode' to None
This should not introduce any behavior changes when using the color extension.
In practive, the colormode will be setup at early at run time to the proper
value (from config and environment).
We do this change as this gets us closer of a state were we can have all the
mechanisms associated to color in core with the feature disabled by default.
Our full contribution guidelines are in our wiki, please see:
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
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).