Merge branch 'jk/pack-delta-reuse-with-bitmap'

When creating a thin pack, which allows objects to be made into a delta against another object that is not in the resulting pack but is known to be present on the receiving end, the code learned to take advantage of the reachability bitmap; this allows the server to send a delta against a base beyond the "boundary" commit. * jk/pack-delta-reuse-with-bitmap: pack-objects: reuse on-disk deltas for thin "have" objects pack-bitmap: save "have" bitmap from walk t/perf: add perf tests for fetches from a bitmapped server t/perf: add infrastructure for measuring sizes t/perf: factor out percent calculations t/perf: factor boilerplate out of test_perf