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()