bug#8612: whitespace-mode for non-unified context diffs

[Chong Yidong]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> Non-unified context diffs use "+ " or even "+\t" at the beginning of
> every line added by the diff (same for removal with - and modifications
> with !).  And whitespace-mode highlights a line that only contains "+ "
> as if it had a trailing space, even though this corresponds to the
> addition of a purely empty line, which should not be highlighted.

Fixed in trunk.