Merge branch 'sg/commit-graph-validate'

Merge branch 'sg/commit-graph-validate'

The code to write commit-graph over given commit object names has
been made a bit more robust.

* sg/commit-graph-validate:
  commit-graph: error out on invalid commit oids in 'write --stdin-commits'
  commit-graph: turn a group of write-related macro flags into an enum
  t5318-commit-graph: use 'test_expect_code'