Fix an issue with "Auditors:" where an edge edit was used as a PHID list

Summary:
See https://discourse.phabricator-community.org/t/runtimeexception-during-import-of-commit/3801. When importing commits with "Auditors:", a raw transaction new value (with an edge edit map using a "+" key) may be passed as an unmentionable PHID list.

Instead, pass an actual PHID list.

Test Plan:

• Pushed a commit with "Auditors: duck".
• Ran daemons.
  • Before patch: umentionable PHID exception.
  • After patch: clean commit import.
• Verified "duck" was added as an auditor.

Differential Revision: https://secure.phabricator.com/D21181