drm_hwcomposer: Use proper commit for author/committer check

The script uses the author/committer of HEAD instead of the commit it is
inspecting. This fails when a patch set has different authors/committers
(such as drm-hwcomposer/drm-hwcomposer!46)

Change-Id: I0fcd724cf372fad435c7614777f13e015c204c3d
Signed-off-by: Sean Paul <seanpaul@chromium.org>