dim: improve the logging from update nightly

Make it all easier to read and understand at a glance. The output will
look something like this:

Updating rerere cache... Done.
Merging origin/drm-intel-next-queued... Reset. Done.
Merging origin/drm-intel-next-fixes... Done.
Merging origin/drm-intel-fixes... Done.
Merging drm-upstream/drm-next... Fast-forward. Done.
Merging drm-upstream/drm-fixes... Using override sha1: d753856c9f9ae33a980192aa7b81d8b97d79dec2... Fast-forward. Done.
Merging origin/topic/core-stuff... Done.
Merging origin/topic/atomic-core... Done.
Merging origin/topic/atomic-helpers... Fast-forward. Done.
Merging sound-upstream/for-next... Done.
Adding integration manifest drm-intel-nightly: 2015y-01m-08d-16h-03m-27s UTC... Done.
Pushing drm-intel-nightly... Done.
Updating rerere cache... Nothing changed. Pushing rerere cache... Done.