branch: avoid unnecessary worktrees traversals

branch: avoid unnecessary worktrees traversals

When we rename a branch ref, we need to update any worktree that have
its HEAD pointing to the branch ref being renamed, so to make it use the
new ref name.

If we know in advance that we're renaming a branch that is not currently
checked out in any worktree, we can skip this step entirely.  Let's do
it so.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>