commit-graph: turn off save_commit_buffer

The commit-graph tool may read a lot of commits, but it only cares about
parsing their metadata (parents, trees, etc) and doesn't ever show the
messages to the user. And so it should not need save_commit_buffer,
which is meant for holding onto the object data of parsed commits so
that we can show them later. In fact, it's quite harmful to do so.
According to massif, the max heap of "git commit-graph write
--reachable" in linux.git before/after this patch (removing the commit
graph file in between) goes from ~1.1GB to ~270MB.

Which isn't surprising, since the difference is about the sum of the
uncompressed sizes of all commits in the repository, and this was
equivalent to leaking them.

This obviously helps if you're under memory pressure, but even without
it, things go faster. My before/after times for that command (without
massif) went from 12.521s to 11.874s, a speedup of ~5%.

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

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