diff: teach --stat to ignore uninteresting modifications

diff: teach --stat to ignore uninteresting modifications

When options such as --ignore-space-change are in use, files with
modifications can have no interesting textual changes worth showing.  In
such cases, "git diff --stat" shows 0 lines of additions and deletions.
Teach "git diff --stat" not to show such a path in its output, which
would be more natural.

However, we don't want to prevent the display  of all files that have 0
effective diffs since they could be the result of a rename, permission
change, or other similar operation that may still be of interest so we
special case additions and deletions as they are always interesting.

Signed-off-by: Matthew Rogers <mattr94@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>