commit-graph: stop using signed integers to count Bloom filters

commit-graph: stop using signed integers to count Bloom filters

When writing a new commit graph we have a couple of counters that
provide statistics around what kind of Bloom filters we have or have not
written. These counters naturally count from zero and are only ever
incremented, but they use a signed integer as type regardless.

Refactor those fields to be unsigned instead. Using an unsigned type
makes it explicit to the reader that they never have to worry about
negative values and thus makes the code easier to understand.

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