commit-graph: fix sign comparison warnings

commit-graph: fix sign comparison warnings

The "commit-graph.c" file has a bunch of sign comparison warnings:

  - There are a bunch of variables that are declared as signed integers
    even though they are used to count entities, like for example
    `num_commit_graphs_before` and `num_commit_graphs_after`.

  - There are several cases where we use signed loop variables to
    iterate through an unsigned entity count.

  - In `write_graph_chunk_base_1()` we count how many chunks we have
    written in total. But while the value represents a positive
    quantity, we still return a signed integer that we then later
    compare with unsigned values.

  - The Bloom settings hash version is being assigned `-1` even though
    it's an unsigned value. This is used to indicate an unspecified
    value and relies on 1's complement.

Fix all of these cases by either using the proper variable type or by
adding casts as required.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>