Merge branch 'rj/branch-in-use-error-message'

A message written in olden time prevented a branch from getting
checked out saying it is already checked out elsewhere, but these
days, we treat a branch that is being bisected or rebased just like
a branch that is checked out and protect it.  Rephrase the message
to say that the branch is in use.

* rj/branch-in-use-error-message:
  branch: error message checking out a branch in use
  branch: error message deleting a branch in use

diff --cc builtin/branch.c
Simple merge

diff --cc t/t2400-worktree-add.sh
Simple merge