checkout: refer to other-worktree branch, not ref

checkout: refer to other-worktree branch, not ref

We can only check out commits or branches, not refs in general.  And the
problem here is if another worktree is using the branch that we want to
check out.

Let’s be more direct and just talk about branches instead of refs.

Also replace “be held” with “in use”.  Further, “in use” is not
restricted to a branch being checked out (e.g. the branch could be busy
on a rebase), hence generalize to “or otherwise in use” in the option
description.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>