dim: log git --version in rerere cache commits

Make it easier to debug potential issues in rerere cache
handling. Adding the commit message using a temp file also makes it
easier to add more breadcrumbs in the future.

Very-much-acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>