Cope with empty initial diff lines

Cope with empty initial diff lines

Some malformed RCS files can have an inital empty line in the text block
for diffs, e.g.

    text
    @
    d1 1
    @

instead of

    text
    @d1 1
    @

Survive this by skipping an initial empty line, but alert the user so
they know something fishy is going on.