httpss://unity3d.com/support/documentation/Manual/ExternalVersionControlSystemSupport.html

Unity offers an Asset Server add-on product for easy integrated versioning of your projects. If you for some reason are not able use the Unity Asset Server, it is possible to store your project in any other version control system, such as Subversion, Perforce or Bazaar, although this requires some manual initial setup of your project and moving and renaming of assets has to be performed using your version control client and not inside Unity.

External Version Control is a Unity Pro feature.