dim: Avoid false positives with BUG detection

Makes sure that the diff line adding the BUG is not immediately
preceded by the diff line removing the BUG. Or in other words,
avoids false positives when existing BUG is edited.

v2: Sent the incomplete version out...

v3: Rebase.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>