Return a handle for the current buffer's state, for a change group.
If you specify BUFFER, make a handle for BUFFER's state instead.
Pass the handle to `activate-change-group' afterward to initiate
the actual changes of the change group.
To finish the change group, call either `accept-change-group' or
`cancel-change-group' passing the same handle as argument. Call
`accept-change-group' to accept the changes in the group as final;
call `cancel-change-group' to undo them all. You should use
`unwind-protect' to make sure the group is always finished. The call
to `activate-change-group' should be inside the `unwind-protect'.
Once you finish the group, don't use the handle again--don't try to
finish the same group twice. For a simple example of correct use, see
the source code of `atomic-change-group'.
The handle records only the specified buffer. To make a multibuffer
change group, call this function once for each buffer you want to
cover, then use `nconc' to combine the returned values, like this:
(nconc (prepare-change-group buffer-1)
(prepare-change-group buffer-2))
You can then activate that multibuffer change group with a single
call to `activate-change-group' and finish it with a single call
to `accept-change-group' or `cancel-change-group'.