Compare this buffer with a file, and highlight differences.
If the buffer has a backup filename, it is used as the default when
this function is called interactively.
If the current buffer is visiting the file being compared against, it
also will have its differences highlighted. Otherwise, the file is
read in temporarily but the buffer is deleted.
If the 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 FILE-B)