28.1.13 Customizing VC
The variable vc-handled-backends determines which version
control systems VC should handle. The default value is (RCS CVS SVN SCCS SRC Bzr Git Hg Mtn), so it contains all the version systems
that are currently supported. If you want VC to ignore one or more of
these systems, exclude its name from the list. To disable VC
entirely, set this variable to nil.
The order of systems in the list is significant: when you visit a
file registered in more than one system, VC uses the system that comes
first in vc-handled-backends by default. The order is also
significant when you register a file for the first time
(see Registering a File for Version Control).