dim: robustify drm-rerere update logic

We've now had two reports (from Chris and Sean each one) that dim
stumbled over uncommitted state in drm-rerere/. In both cases a bunch
of git rm have been staged, but not committed.

I didn't really figure out what exactly happend, but best explanation
is that one of the commands failed, but silently, since we &>
/dev/null the output. Make sure no matter what happens, we get at
least to the commit step to not leave a broken state behind and add a
|| true everywhere it's missing.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>