"This is done by selecting Edit->Project Settings->Editor in the application menu and enabling External Version Control support by selecting Visible Meta Files in the dropdown for Version Control. This will show a text file for every asset in the Assets directory containing the necessary bookkeeping information required by Unity. The files will have a .meta file extension with the first part being the full file name of the asset it is associated with. Moving and renaming assets within Unity should also update the relevant .meta files. However, if you move or rename assets from an external tool, make sure to syncronize the relevant .meta files as well."

http://docs.unity3d.com/Manual/ExternalVersionControlSystemS...