commit-graph: always parse before commit_graph_data_at()

commit-graph: always parse before commit_graph_data_at()

There is a subtle failure happening when computing corrected commit
dates with --split enabled. It requires a base layer needing the
generation_data_overflow chunk. Then, the next layer on top
erroneously thinks it needs an overflow chunk due to a bug leading
to recalculating all reachable generation numbers. The output of
the failure is

  BUG: commit-graph.c:1912: expected to write 8 bytes to
  chunk 47444f56, but wrote 0 instead

These "expected" 8 bytes are due to re-computing the corrected
commit date for the lower layer but the new layer does not need
any overflow.

Add a test to t5318-commit-graph.sh that demonstrates this bug. However,
it does not trigger consistently with the existing code.

The generation number data is stored in a slab and accessed by
commit_graph_data_at(). This data is initialized when parsing a commit,
but is otherwise used assuming it has been populated. The loop in
compute_generation_numbers() did not enforce that all reachable
commits were parsed and had correct values. This could lead to some
problems when writing a commit-graph with corrected commit dates based
on a commit-graph without them.

It has been difficult to identify the issue here because it was so hard
to reproduce. It relies on this uninitialized data having a non-zero
value, but also on specifically in a way that overwrites the existing
data.

This patch adds the extra parse to ensure the data is filled before we
compute the generation number of a commit. This triggers the new test
to fail because the generation number overflow count does not match
between this computation and the write for that chunk.

The actual fix will follow as the next few changes.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>