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