branch: fix die_if_checked_out() when ignore_current_worktree

In 8d9fdd7 (worktree.c: check whether branch is rebased in another worktree, 2016-04-22) die_if_checked_out() learned a new option ignore_current_worktree, to modify the operation from "die() if the branch is checked out in any worktree" to "die() if the branch is checked out in any worktree other than the current one". Unfortunately we implemented it by checking the flag is_current in the worktree that find_shared_symref() returns. When the same branch is checked out in several worktrees simultaneously, find_shared_symref() will return the first matching worktree in the list composed by get_worktrees(). If one of the worktrees with the checked out branch is the current worktree, find_shared_symref() may or may not return it, depending on the order in the list. Instead of find_shared_symref(), let's do the search using use the recently introduced API is_shared_symref(), and consider ignore_current_worktree when necessary. Signed-off-by: Rubén Justo <rjusto@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>