Merge branch 'tb/commit-graph-harden'

The code to parse and use the commit-graph file has been made more
robust against corrupted input.

* tb/commit-graph-harden:
  commit-graph.c: handle corrupt/missing trees
  commit-graph.c: handle commit parsing errors
  t/t5318: introduce failing 'git commit-graph write' tests

diff --cc commit-graph.c
Simple merge

diff --cc commit.c
Simple merge

diff --cc t/t5318-commit-graph.sh
Simple merge