R1:3aed39b8b072

Fix an issue with serializing reviewers over the wire

Fixes T10981. Ref T10939. `arc` currently has some odd, hard-coded checks
(missing reviewers, all reviewers away) that depend on the field value being
in a certain format.

The recent changes swapped the field value from scalars (PHIDs) to
dictionaries and broke this workflow. It worked fine in testing because we
apply these checks very inconsistently (not on update or `--edit`).

To get around this for now, serialize into "PHID!" and then unserialize…