
Just wondering which tool generated this git tree view? Thanks.
This is gitx, as suggested in other answers. However, this graph is not an achievement of this tool only; such a view can be generated by many tools (qgit, tortoisegit, gitg), including bare git itself output to console!
Can be any of gitk, gitextensions, tortoisegit, and many more.
Looks like gitx http://gitx.frim.nl/seeit.html
 
    
    You can also use git log --graph to see the graphical view of commits, very similar to what is shown above in the picture, but from commandline, and without installing any additional software
