Merge branch 'tb/midx-with-changing-preferred-pack-fix'

Merge branch 'tb/midx-with-changing-preferred-pack-fix'

Multi-pack index got corrupted when preferred pack changed from one
pack to another in a certain way, which has been corrected.

* tb/midx-with-changing-preferred-pack-fix:
  midx.c: avoid adding preferred objects twice
  midx.c: include preferred pack correctly with existing MIDX
  midx.c: extract `midx_fanout_add_pack_fanout()`
  midx.c: extract `midx_fanout_add_midx_fanout()`
  midx.c: extract `struct midx_fanout`
  t/lib-bitmap.sh: avoid silencing stderr
  t5326: demonstrate potential bitmap corruption