worktree: fix "add -B"

Current code does not update "symref" when -B is used. This string
contains the new HEAD. Because it's empty "git worktree add -B" fails at
symbolic-ref step.

Because branch creation is already done before calling add_worktree(),
-B is equivalent to -b from add_worktree() point of view. We do not need
the special case for -B.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/worktree.c

@@ -201,9 +201,7 @@ static int add_worktree(const char *path, const char *refname,
 		die(_("'%s' already exists"), path);
 
 	/* is 'refname' a branch or commit? */
-	if (opts->force_new_branch) /* definitely a branch */
-		;
-	else if (!opts->detach && !strbuf_check_branch_ref(&symref, refname) &&
+	if (!opts->detach && !strbuf_check_branch_ref(&symref, refname) &&
 		 ref_exists(symref.buf)) { /* it's a branch */
 		if (!opts->force)
 			die_if_checked_out(symref.buf);