worktree add -B: do the checkout test before update branch

If --force is not given but -B is, we should not proceed if the given
branch is already checked out elsewhere. add_worktree() has this test,
but it kicks in too late when "git branch --force" is already
executed. As a result, even though we correctly refuse to create a new
worktree, we have already updated the branch and mess up the other
checkout.

Repeat the die_if_checked_out() test again for this specific case before
"git branch" runs.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/worktree.c b/builtin/worktree.c
index 6b9c946..20cf67a 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -334,9 +334,18 @@
 	branch = ac < 2 ? "HEAD" : av[1];
 
 	opts.force_new_branch = !!new_branch_force;
-	if (opts.force_new_branch)
+	if (opts.force_new_branch) {
+		struct strbuf symref = STRBUF_INIT;
+
 		opts.new_branch = new_branch_force;
 
+		if (!opts.force &&
+		    !strbuf_check_branch_ref(&symref, opts.new_branch) &&
+		    ref_exists(symref.buf))
+			die_if_checked_out(symref.buf);
+		strbuf_release(&symref);
+	}
+
 	if (ac < 2 && !opts.new_branch && !opts.detach) {
 		int n;
 		const char *s = worktree_basename(path, &n);