Return BUFFER's character-change tick counter.
Each buffer has a character-change tick counter, which is set to the
value of the buffer's tick counter (see `buffer-modified-tick'), each
time text in that buffer is inserted or deleted. By comparing the
values returned by two individual calls of `buffer-chars-modified-tick',
you can tell whether a character change occurred in that buffer in
between these calls. No argument or nil as argument means use current
buffer as BUFFER. (fn &optional BUFFER)