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

diff --cc builtin/branch.c
Simple merge

diff --cc builtin/clone.c
Simple merge

diff --cc remote.c
Simple merge

diff --cc t/test-lib-functions.sh
Simple merge