I'd like to see who contributed which line/change to a file. git-blame does exactly that. So does SourceTree have a git-blame view?
Select a file, whether in the 'working copy' display or in the 'commit' display, and do one of three things:
If the file you want to blame is not in the modified files list, you need to change the view to File Status View (View --> File Status View), then set the Sub menu to "Show All"