chunk-format: introduce `pair_chunk_expect()` helper

In 570b8b8836 (chunk-format: note that pair_chunk() is unsafe,
2023-10-09), the pair_chunk() interface grew a required "size" pointer,
so that the caller is forced to at least have a handle on the actual
size of the given chunk.

Many callers were converted to the new interface. A handful were instead
converted from the unsafe version of pair_chunk() to read_chunk() so
that they could check their expected size.

This led to a lot of code like:

    static int graph_read_oid_lookup(const unsigned char *chunk_start,
                                     size_t chunk_size, void *data)
    {
      struct commit_graph *g = data;
      g->chunk_oid_lookup = chunk_start;
      if (chunk_size / g->hash_len != g->num_commits)
        return error(_("commit-graph OID lookup chunk is the wrong size"));
      return 0;
    }

, leaving the caller to use read_chunk(), like so:

    read_chunk(cf, GRAPH_CHUNKID_OIDLOOKUP, graph_read_oid_lookup, graph);

The callback to read_chunk() (in the above, `graph_read_oid_lookup()`)
does nothing more than (a) assign a pointer to the location of the start
of the chunk in the mmap'd file, and (b) assert that it has the correct
size.

For callers that know the expected size of their chunk(s) up-front (most
often because they are made up of a known number of fixed-size records),
we can simplify this by teaching the chunk-format API itself to validate
the expected size for us.

This is wrapped in a new function, called `pair_chunk_expect()` which
takes a pair of "size_t"s (corresponding to the record size and count),
instead of a "size_t *", and validates that the given chunk matches the
expected size as given.

This will allow us to reduce the number of lines of code it takes to
perform these basic read_chunk() operations, by taking the above and
replacing it with something like:

    if (pair_chunk_expect(cf, GRAPH_CHUNKID_OIDLOOKUP,
                          &graph->chunk_oid_lookup,
                          graph->hash_len, graph->num_commits))
      error(_("commit-graph oid lookup chunk is wrong size"));

We will perform those transformations in the following commits.

Helped-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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