commit-graph.c: prevent overflow in `fill_commit_graph_info()`

In a similar spirit as previous commits, ensure that we don't overflow
in a few spots within `fill_commit_graph_info()`:

  - First, when computing an offset into the commit data chunk, which
    can occur when the `lex_index` of the item we're looking up exceeds
    2^32-1/GRAPH_DATA_WIDTH.

  - A similar issue when computing the generation date offset for
    commits with `lex_index` greater than 2^32-1/4. Note that in
    practice this will never overflow, since the left-hand operand is
    from calling `sizeof(...)` and is thus already a `size_t`. But wrap
    that in an `st_mult()` to make it clear that we intend to perform
    this computation using 64-bit operands.

  - Finally, a nearly identical issue as above when computing an offset
    into the `generation_data_overflow` chunk.

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

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