branch: use get_worktrees() in copy_or_rename_branch()

branch: use get_worktrees() in copy_or_rename_branch()

Obtaining the list of worktrees, using get_worktrees(), is not a
lightweight operation, because it involves reading from disk.

Let's stop calling get_worktrees() in reject_rebase_or_bisect_branch()
and in replace_each_worktree_head_symref().  Make them receive the list
of worktrees from their only caller, copy_or_rename_branch().

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