R1:bc41c3f5a5b9

Use DifferentialCommitMessageParser and Modular Transactions to implement "Auditors: ..."

Summary:
Ref T10978. Updates how we implement "Auditors: ..." in commit messages:

- Use the same parsing code as everything else.
- (Also: parse package names.)
- Use the new transaction code.

Also, fix some UI strings.

Test Plan: Used `bin/repository reparse --herald <commit>` to re-run this code on commits with various messages (valid Auditors, invalid Auditors, no Auditors). Saw appropriate auditors…