Compare two buffers and highlight the differences.
The default is the current buffer and the one in the next window.
If either buffer is modified and is visiting a file, you are prompted
to save the file.
Unless the buffer is unmodified and visiting a file, the buffer is
written to a temporary file for comparison.
If a buffer is read-only, differences will be highlighted but no property
changes are made, so M-x highlight-changes-next-change and
M-x highlight-changes-previous-change will not work. (fn BUF-A BUF-B)