Command log improvements: time measurements

For performance evaluation of huge Git repositories we've improved GitExtensions' git command log with additional time data. Take a look, hopefully we can see it in the next version of GitExtensions :-)

![image](https://cloud.githubusercontent.com/assets/1516470/3469012/c6711c08-02a7-11e4-87d4-eecbe2d10c7d.png)

Best regards,
Dominik