Unity’s Asset Serialization Mode controls how meta files and Scenes are stored on your hard drive. You should set Unity to use the Force Text setting for Asset Serialization Mode when working with a VCS.
In this mode, Unity stores Assets and Scenes in a text-based YAML format. This makes it possible to merge conflicts between differing versions of the file.
At the time of writing (February 2019) Unity uses Force Text as the default for new projects.
To set this mode for legacy projects chose Edit → Project Settings → Editor from the Unity Editor’s application menu. Then choose Force Text from the Asset Serialization Mode dropdown.