Remove extraneous empty lines around diff headers

Before:

    --- /tmp/foo

    +++ <blockwart content>

    @@ -1,2 +1,2 @@

    ...

After:

    --- /tmp/foo
    +++ <blockwart content>
    @@ -1 +1 @@
    ...

(Line numbers were off by one, too, but nobody reads them anyway.)