Merge branch 'jk/commit-graph-verify-fix'

Various fixes to "git commit-graph verify".

* jk/commit-graph-verify-fix:
  commit-graph: report incomplete chains during verification
  commit-graph: tighten chain size check
  commit-graph: detect read errors when verifying graph chain
  t5324: harmonize sha1/sha256 graph chain corruption
  commit-graph: check mixed generation validation when loading chain file
  commit-graph: factor out chain opening function