auto-revert-check-vc-info is a variable defined in `autorevert.el'.
Its value is

If non-nil Auto Revert Mode reliably updates version control info.
Auto Revert Mode updates version control info whenever the buffer
needs reverting, regardless of the value of this variable.
However, the version control state can change without changes to
the work file. If the change is made from the current Emacs
session, all info is updated. But if, for instance, a new
version is checked in from outside the current Emacs session, the
version control number in the mode line, as well as other version
control related information, may not be properly updated. If you
are worried about this, set this variable to a non-nil value.

This currently works by automatically updating the version
control info every `auto-revert-interval' seconds. Nevertheless,
it should not cause excessive CPU usage on a reasonably fast
machine, if it does not apply to too many version controlled
buffers. CPU usage depends on the version control system.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.