Merge branch 'tb/midx-use-checksum'

When rebuilding the multi-pack index file reusing an existing one,
we used to blindly trust the existing file and ended up carrying
corrupted data into the updated file, which has been corrected.

* tb/midx-use-checksum:
  midx: report checksum mismatches during 'verify'
  midx: don't reuse corrupt MIDXs when writing
  commit-graph: rewrite to use checksum_valid()
  csum-file: introduce checksum_valid()

diff --cc csum-file.c
Simple merge

diff --cc csum-file.h
Simple merge