fix gitk command for Windows

The command "Gitk" (`GitGitkCommand`) does not work on Windows (I'm using Windows 7) when all others commands are ok.
This is a pull request to correct this issue. It works for me while not breaking others commands, but I would like to know if other people have this issue and if this patch is also ok for them.

The trick to fix the issue is to set the variable `shell` to `true` on Windows, it is inspired by the SideBarGit plugin for Sublime Text 3 (see https://github.com/SublimeText/SideBarGit/blob/95348b1c0802e4873d5b6fca65706914c52d035d/sidebar/SideBarGit.py#L150-L163).