dim: fix update_rerere_cache

update_rerere_cache does not work correctly if drm-tip and $DIM_REPO are
workdirs - it cannot locate properly rr-cache. Let's use git rev-parse
to resolve rr-cache location.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>