Rewrite the Revision Walker

The new revision walker uses an internal Commit object storage system,
custom memory allocator and much improved topological and time sorting
algorithms. It's about 20x times faster than the previous implementation
when browsing big repositories.

The following external API calls have changed:

	`git_revwalk_next` returns an OID instead of a full commit object.
	The initial call to `git_revwalk_next` is no longer blocking when
	iterating through a repo with a time-sorting mode.

	Iterating with Topological or inverted modes still makes the initial
	call blocking to preprocess the commit list, but this block should be
	mostly unnoticeable on most repositories (topological preprocessing
	times at 0.3s on the git.git repo).

	`git_revwalk_push` and `git_revwalk_hide` now take an OID instead
	of a full commit object.

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