Merge branch 'tb/commit-graph-genv2-upgrade-fix'

There was a bug in the codepath to upgrade generation information
in commit-graph from v1 to v2 format, which has been corrected.

* tb/commit-graph-genv2-upgrade-fix:
  commit-graph: fix corrupt upgrade from generation v1 to v2
  commit-graph: introduce `repo_find_commit_pos_in_graph()`
  t5318: demonstrate commit-graph generation v2 corruption

diff --cc commit-graph.c
Simple merge

diff --cc commit-graph.h
Simple merge

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