Merge branch 'ps/pack-refs-auto-heuristics'

"git pack-refs --auto" for the files backend was too aggressive,
which has been a bit tamed.

* ps/pack-refs-auto-heuristics:
  refs/files: use heuristic to decide whether to repack with `--auto`
  t0601: merge tests for auto-packing of refs
  wrapper: introduce `log2u()`

diff --cc bisect.c
Simple merge

diff --cc refs/files-backend.c
Simple merge

diff --cc refs/packed-backend.c
Simple merge