test: add failing tests for "diff --stat" to t4049

test: add failing tests for "diff --stat" to t4049

There are a few problems in diff.c around --stat area, partially
caused by the recent 74faaa1 (Fix "git diff --stat" for interesting
- but empty - file changes, 2012-10-17), and largely caused by the
earlier change that introduced when --stat-count was added.

Add a few test pieces to t4049 to expose the issues.

Signed-off-by: Junio C Hamano <gitster@pobox.com>