Merge branch 'tb/repack-clearing-midx'

When a packfile is removed by "git repack", multi-pack-index gets cleared; the code was taught to do so less aggressively by first checking if the midx actually refers to a pack that no longer exists. * tb/repack-clearing-midx: midx: traverse the local MIDX first builtin/repack.c: invalidate MIDX only when necessary