http://www.dannygoodayle.com/2013/02/14/using-unity-with-git said:
Within Unity, press Edit > Project Settings > Editor. In your inspector window you will see the Editor Settings for your project, under “Version Control” set the mode to Meta Files.
This will tell Unity to generate a small file for each object in your project (everything; Scripts, Prefabs, Textures, you name it, it metas it). Now, when an asset is modified in the editor, only the asset and it’s associated meta file will report changes, instead of the entire Library folder.
And then Asset Serialization, set it’s mode to “Force Text”. This will output Unity levels as an YAML document instead of a binary file