llvm.git mirror & release_31 branch

Hello Everyone

As some of you already noticed release_31 branch on llvm.git was
somehow created w/o history. I fixed this via manual grafting (kudos
to John Wiegley!), but please carefully watch the results:
1. refs in release_31 branch should be changed. This is expected and
this is the price we're paying for connected history (if you have
something on top you might want to do git pull --rebase)
2. refs in other branches should remain as they were

Please watch the mirror carefully and let me know if there will be any