[RFC] Second parent for reverts

The discussion about having a header to specify, for a revert commit, what it reverts made me realize that this header *would* be useful, but that we don't need a *new* header for it. I think that the right method is to add the parent of the reverted commit as a second parent for the revert.

If you have:

a -> b -> c -> d

And you want to revert b, the most exact flow would be:

a -> b -> c -> d -> e
       \         /
        -> a' ---

I.e., you exactly remove the effects of b to generate a commit that has the same tree as a, and then you merge.

But a' doesn't actually take anything from b, since it's reverting all of b (unless it's only reverting part of b), and, if b isn't there, it doesn't need a commit message, either, so it's not different from a. So the flow should be:

a -> b -> c -> d -> e
  \              /
   --------------

And this means blame work correctly: lines that b changed will be blamed on a (or an ancestor), because e will match a there and be different from d. So I think git-revert should simply add in the reverted patch's parent. Does this analysis make sense to other people?

	-Daniel
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