config: use the new '_unset' value for 'configlist'
This should let 'configlist' delegate all special processing of the default
config value to the main 'config' method.
The default config value ([]) is still handled in this method.
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https://www.mercurial-scm.org/wiki/ContributingChanges
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273ce12ad8f1` will report some style violations on a very old commit).