Diff mode

[Richard Stallman]

    From: Paul Eggert <address@hidden>

    The current POSIX revision specifies the behavior of "diff -c", but
    not of "diff -u".  But so many people now use -u format that the next
    POSIX revision will specify it.  The draft POSIX specification for -u
    says that "diff -u" format can use an empty line instead of a line
    with a single space.  This was in response to Jim's concern.

So we need to change Diff mode, even though I asked them to revert the
change in GNU Diff.

Would someone please make that change in Diff mode, then ack?