Compare text in current window with text in next window.
Compares the text starting at point in each window,
moving over text in each one as far as they match.
This command pushes the mark in each window
at the prior location of point in that window.
If both windows display the same buffer,
the mark is pushed twice in that buffer:
first in the other window, then in the selected window.
A prefix arg means reverse the value of variable
`compare-ignore-whitespace'. If `compare-ignore-whitespace' is
nil, then a prefix arg means ignore changes in whitespace. If
`compare-ignore-whitespace' is non-nil, then a prefix arg means
don't ignore changes in whitespace. The variable
`compare-windows-whitespace' controls how whitespace is skipped.
If `compare-ignore-case' is non-nil, changes in case are also
ignored.
If `compare-windows-sync' is non-nil, then successive calls of
this command work in interlaced mode:
on first call it advances points to the next difference,
on second call it synchronizes points by skipping the difference,
on third call it again advances points to the next difference and so on.
(fn IGNORE-WHITESPACE)