Merge branch 'ky/branch-m-worktree' into maint

When "git worktree" feature is in use, "git branch -m" renamed a
branch that is checked out in another worktree without adjusting
the HEAD symbolic ref for the worktree.

* ky/branch-m-worktree:
  set_worktree_head_symref(): fix error message
  branch -m: update all per-worktree HEADs
  refs: add a new function set_worktree_head_symref

diff --cc builtin/branch.c
Simple merge

diff --cc t/t3200-branch.sh
Simple merge