commit-graph: fix corrupt upgrade from generation v1 to v2

The previous commit demonstrates a bug where a commit-graph using
generation v2 could enter a state where one of the GDA2 values has its
most-significant bit set (indicating that its value should be read from
the extended offset table in the GDO2 chunk) without having a GDO2 chunk
to read from.

This results in the following error message being displayed to the
caller:

    fatal: commit-graph requires overflow generation data but has none

This bug arises in the following scenario:

  - We decide to write a commit-graph using generation number v2, and
    decide (correctly) that no GDO2 chunk is necessary (e.g., because
    all of the commiter date offsets are no larger than 2^31-1).

  - The v2 generation numbers are stored in the `->generation` member of
    the commit slab holding `struct commit_graph_data`'s.

  - Later on, `load_commit_graph_info()` is called, overwriting the
    v2 generation data in the aforementioned slab with any existing v1
    generation data.

Then, when the commit-graph code goes to write the GDA2 chunk via
`write_graph_chunk_generation_data()`, we use the overwritten generation
v1 data in a place where we expect to use a v2 generation number:

    offset = commit_graph_data_at(c)->generation - c->date;

...because `commit_graph_data_at(c)->generation` used to hold the v2
generation data, but it was overwritten to contain the v1 generation
number via `load_commit_graph_info()`.

If the `offset` computation above overflows the v2 generation number
max, then `write_graph_chunk_generation_data()` will update its count of
large offsets and write the marker accordingly:

    if (offset > GENERATION_NUMBER_V2_OFFSET_MAX) {
        offset = CORRECTED_COMMIT_DATE_OFFSET_OVERFLOW | num_generation_data_overflows;
        num_generation_data_overflows++;
    }

and reads will look for the GDO2 chunk containing the overflowing v2
generation number, *after* the commit-graph code decided that no such
chunk was necessary.

The main problem is that the slab containing `struct commit_graph_data`
has a dual purpose. It is used to hold data that we are about to write
to disk while generating a commit-graph, as well as hold data that was
read from an existing commit-graph.

When the two mix, namely when the result of reading the commit-graph has
a side-effect that mixes poorly with an in-progress commit-graph write,
we end up with corrupt data.

A complete fix might be to introduce a new slab that is used exclusively
for writing, and gate access between the two slabs based on context
provided by the caller (e.g., whether this computation is part of a
"read" or "write" operation).

But a more minimal fix addresses the only known path which overwrites
the slab data, which is `compute_bloom_filters()` ->
`get_or_compute_bloom_filter()` -> `load_commit_graph_info()` ->
`fill_commit_graph_info()` by avoiding the last call which clobbers the
data altogether.

This path only needs to learn the graph position of a given commit so
that it can be used in `load_bloom_filter_from_graph()`. By replacing
the last steps of the above with one that records the graph position
into a temporary variable which is then used to load the existing Bloom
data, we eliminate the clobbering, removing the corruption.

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