Merge branch 'tb/midx-write-propagate-namehash'

Merge branch 'tb/midx-write-propagate-namehash'

"git multi-pack-index write --bitmap" learns to propagate the
hashcache from original bitmap to resulting bitmap.

* tb/midx-write-propagate-namehash:
  t5326: test propagating hashcache values
  p5326: generate pack bitmaps before writing the MIDX bitmap
  p5326: don't set core.multiPackIndex unnecessarily
  p5326: create missing 'perf-tag' tag
  midx.c: respect 'pack.writeBitmapHashcache' when writing bitmaps
  pack-bitmap.c: propagate namehash values from existing bitmaps
  t/helper/test-bitmap.c: add 'dump-hashes' mode