Fix regexp pattern searching git-dir

1. '/.git' is unnecessary under submodule directory
   ex)
   Parent repository: C:\home\dotfiles
   Submodule repository: C:\home\dotfiles\.git\modules\dotfiles\.vim\bundle\restart.vim\COMMIT_EDITMSG
   Submodule path: C:\home\dotfiles\dotfiles\.vim\bundle\restart.vim
2. Support 'noshellslash'.
fix git-dir searching pattern again (#5)

Thanks to @rhysd & @Flast
Merge pull request #5 from tyru/fix-search-git-root

Fix regexp pattern searching git-dir (via #4)