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