Merge branch 'jk/rev-list-verify-objects-fix'

Merge branch 'jk/rev-list-verify-objects-fix'

"git rev-list --verify-objects" ought to inspect the contents of
objects and notice corrupted ones, but it didn't when the commit
graph is in use, which has been corrected.

* jk/rev-list-verify-objects-fix:
  rev-list: disable commit graph with --verify-objects
  lookup_commit_in_graph(): use prepare_commit_graph() to check for graph