handle git-notes breakage

handle git-notes breakage

The new git-notes feature in git 1.6.6 changes git log output in a way that
broke ikiwiki's parser if notes are added to commits.

I decided to deal with this by disabling notes when ikiwiki uses git,
by setting GIT_NOTES_REF="". AFAICS, looking up notes when dumping logs
will only waste time, since it does not currently seem to make sense for
ikiwiki to do anything with the notes.