"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
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