Merge branch 'js/init-defaultbranch-advice'

Our users are going to be trained to prepare for future change of init.defaultBranch configuration variable. * js/init-defaultbranch-advice: init: provide useful advice about init.defaultBranch get_default_branch_name(): prepare for showing some advice branch -m: allow renaming a yet-unborn branch init: document `init.defaultBranch` better