[PATCH 0/3] Teach git-blame about renames (take 2)

Changes since the previous version:

* Fix the things pointed out by Junio.
* Some other minor clean-ups

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This patch series teaches git-blame about renames. To do this I have changed the revision.h interface a bit. In particular, it is now possible for the user of revision.h to specify a try_to_simply_commit-like function. That function can then do the rename tracking.

I have also made a small change to sort_in_topological_order to make it possible to use the object.util field at the same time as a topological sort is done. Previously the object.util field was clobbered by the topological sort. In the new interface the auxiliary data that the topological sort needs to store for each commit object is stored with a setter function and retrieved by a getter. Pointers to those functions are passed to sort_in_topological_order_fn.

- Fredrik