In the last few versions, SourceTree introduced a new "feature" which I guess you thought would make it easier to read rows of lines.

Unfortunately, the colour/zebra striping is too loud and actually makes it more difficult to read each row without being distracted by the colour scheme!

I would suggest either radically lightening-down the gray colour, or simply add a faint border under each row to distinguish the lines.

Please see the following article on whether zebra striping works: -
http://alistapart.com/article/zebrastripingdoesithelp

Also, take a look at the following screenshot on how Apple implements it lightly in iTunes: -
http://images.apple.com/itunes/features/images/features_player_organize_screen.jpg