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

diff --cc builtin/repack.c
Simple merge

diff --cc midx.c
Simple merge

diff --cc t/t5319-multi-pack-index.sh
Simple merge