include etc/motd in portable git

Taken from [share/WinGit/release.sh](https://github.com/msysgit/msysgit/blob/master/share/WinGit/release.sh).

Before:
![1](https://cloud.githubusercontent.com/assets/1192780/4878676/e190f2b4-6315-11e4-9c37-55a964e1e4bd.PNG)

After:
![2](https://cloud.githubusercontent.com/assets/1192780/4878681/102e57ce-6316-11e4-8bba-3aa5b2d3e0af.PNG)