Merge branch 'tb/pack-preferred-tips-to-give-bitmap'

A configuration variable has been added to force tips of certain
refs to be given a reachability bitmap.

* tb/pack-preferred-tips-to-give-bitmap:
  builtin/pack-objects.c: respect 'pack.preferBitmapTips'
  t/helper/test-bitmap.c: initial commit
  pack-bitmap: add 'test_bitmap_commits()' helper

diff --cc Makefile
Simple merge

diff --cc pack-bitmap.c
Simple merge

diff --cc t/helper/test-tool.c
Simple merge

diff --cc t/helper/test-tool.h
Simple merge