Function to check whether a non-file buffer needs reverting.
This should be a function with one optional argument NOCONFIRM.
Auto Revert Mode passes t for NOCONFIRM. The function should return
non-nil if the buffer should be reverted. A return value of
`fast' means that the need for reverting was not checked, but
that reverting the buffer is fast. The buffer is current when
this function is called.
The idea behind the NOCONFIRM argument is that it should be
non-nil if the buffer is going to be reverted without asking the
user. In such situations, one has to be careful with potentially
time consuming operations.
For more information on how this variable is used by Auto Revert mode,
see Info node `(emacs)Supporting additional buffers'.