fix unite-grep.

If user specified directory name on "Target", Unite can't open
candidates file. I fixed this befavior.
Improved a fix.

This fix change the current directory before grep, but the behavior is
the same.
Merge pull request #176 from yohe/master

unite-grep： Target にディレクトリを指定した場合に、 候補ファイルを開くことができない