Add tgit.icon in .tgitconfig

This is a new feature in TortoiseGit 1.8.6,
which allows to show SherpDevelop icon as taskbar overlay icon

![tgit icon-help](https://f.cloud.github.com/assets/791115/1322125/1325c0b6-3414-11e3-8331-b2259e6ee61c.png)