git/stats: Implement reading bitmap info

Git has recently introduced support for bitmap lookup tables in Git v2.38.0. This extension allows Git to defer loading bitmaps until they're really required, which should speed up especially large repos that have a bunch of bitmaps.

As a first step towards enabling this extension we'll start to expose information about bitmaps via our housekeeping machinery so that we can monitor the rollout and whether repositories get converted to use the new extension as expected.

Implement logic to read bitmaps in the git/stats package and execute it when reading packfiles. This causes us to log information about bitmaps during housekeeping operations.

Part of #4734 (closed).