commit-graph: avoid using stale stack addresses

commit-graph: avoid using stale stack addresses

The code is a bit too hard to reason about to fully assess whether the
`fill_commit_graph_info()` function is called at all after
`write_commit_graph()` returns (and hence the stack variable
`topo_levels` goes out of context).

Let's simply make sure that the stack address is no longer used at that
stage, thereby making the code quite a bit easier to reason about.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>