pack-bitmap-write: reimplement bitmap writing

The bitmap generation code works by iterating over the set of commits
for which we plan to write bitmaps, and then for each one performing a
traditional traversal over the reachable commits and trees, filling in
the bitmap. Between two traversals, we can often reuse the previous
bitmap result as long as the first commit is an ancestor of the second.
However, our worst case is that we may end up doing "n" complete
complete traversals to the root in order to create "n" bitmaps.

In a real-world case (the shared-storage repo consisting of all GitHub
forks of chromium/chromium), we perform very poorly: generating bitmaps
takes ~3 hours, whereas we can walk the whole object graph in ~3
minutes.

This commit completely rewrites the algorithm, with the goal of
accessing each object only once. It works roughly like this:

  - generate a list of commits in topo-order using a single traversal

  - invert the edges of the graph (so have parents point at their
    children)

  - make one pass in reverse topo-order, generating a bitmap for each
    commit and passing the result along to child nodes

We generate correct results because each node we visit has already had
all of its ancestors added to the bitmap. And we make only two linear
passes over the commits.

We also visit each tree usually only once. When filling in a bitmap, we
don't bother to recurse into trees whose bit is already set in the
bitmap (since we know we've already done so when setting their bit).
That means that if commit A references tree T, none of its descendants
will need to open T again. I say "usually", though, because it is
possible for a given tree to be mentioned in unrelated parts of history
(e.g., cherry-picking to a parallel branch).

So we've accomplished our goal, and the resulting algorithm is pretty
simple to understand. But there are some downsides, at least with this
initial implementation:

  - we no longer reuse the results of any on-disk bitmaps when
    generating. So we'd expect to sometimes be slower than the original
    when bitmaps already exist. However, this is something we'll be able
    to add back in later.

  - we use much more memory. Instead of keeping one bitmap in memory at
    a time, we're passing them up through the graph. So our memory use
    should scale with the graph width (times the size of a bitmap).

So how does it perform?

For a clone of linux.git, generating bitmaps from scratch with the old
algorithm took 63s. Using this algorithm it takes 205s. Which is much
worse, but _might_ be acceptable if it behaved linearly as the size
grew. It also increases peak heap usage by ~1G. That's not impossibly
large, but not encouraging.

On the complete fork-network of torvalds/linux, it increases the peak
RAM usage by 40GB. Yikes. (I forgot to record the time it took, but the
memory usage was too much to consider this reasonable anyway).

On the complete fork-network of chromium/chromium, I ran out of memory
before succeeding. Some back-of-the-envelope calculations indicate it
would need 80+GB to complete.

So at this stage, we've managed to make things much worse. But because
of the way this new algorithm is structured, there are a lot of
opportunities for optimization on top. We'll start implementing those in
the follow-on patches.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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