Add default arg to Diff ctor

Change-Id: I97d4b99250d55216ef866f7529ca423e2133db09
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Fix return arg of Diff class getter

Change-Id: I25dc6d2992831d79077e4d06f52018d6191e0bfb
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>