R1:d84c0a5be59c

Validate all fields in differential.parsecommitmessage

Summary:
- We currently run ##parseValueFromCommitMessage()## on all fields present in
the message, but not ##validateField()##.
- This detects value errors (e.g., an invalid reviewer) but not higher-level
errors (e.g., a missing field).
- This can break the stacked-commits Git mutable history workflow by
recognizing too many commit messages as valid ("multiple valid commit messages,
this is ambiguous").
- This also gives you some errors…