Make "diff --check" output match "git apply"

Make "diff --check" output match "git apply"

For consistency, make the two tools report whitespace errors in the
same way (the output of "diff --check" has been tweaked to match
that of "git apply").

Note that although the textual content is basically the same only
"git diff --check" provides a colorized version of the problematic
lines; making "git apply" do colorization will require more extensive
changes (figuring out the diff colorization preferences of the user)
and so that will be a subject for another commit.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>