Fix bug that happens when use -resume with -start-insert.

* Problems summary

When reusing a unite buffer, with the `start-insert` option set, sometimes the cursor is placed before the prompt sign.

* Expected

With `start-insert` option set, when reusing a unite buffer, always place the cursor at the end of the prompt line.

* Environment Information
 * OS: OS X Mavericks.
 * Vim version:

```console
$ vim --version
VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Jul 16 2014 09:51:29)
MacOS X (unix) version
Included patches: 1-367
```

* Minimal vimrc less than 50 lines

```VimL
" minimal.vimrc
if has('vim_starting')
  set nocompatible
  set runtimepath=~/.vim/bundle/unite.vim/
endif
```

* How to reproduce

 Here is a gif showing the bug:

![unite](https://cloud.githubusercontent.com/assets/120483/3615910/0ef17b32-0dcd-11e4-9bc6-065fc575079a.gif)