Fix DiffTreeVertexDiffTerm definition to be a valid protobuf object.

[patrickt]

We define the DiffTreeVertex protobuf message like so:

message DiffTreeVertex {
  int32 diff_vertex_id = 1;
  oneof diff_term {
    DeletedTerm deleted = 2;
    InsertedTerm inserted = 3;
    ReplacedTerm replaced = 4;
    MergedTerm merged = 5;
  }
}

This is turned into two Haskell types, a toplevel DiffTreeVertex type
and a DiffTreeVertexDiffTerm type that represents the anonymous
oneof type. Said types looked like so:

data DiffTreeVertexDiffTerm
  = Deleted (Maybe DeletedTerm)
  | Inserted (Maybe InsertedTerm)
  | Replaced (Maybe ReplacedTerm)
  | Merged (Maybe MergedTerm)
  deriving stock (Eq, Ord, Show, Generic)
  deriving anyclass (Proto3.Message, Proto3.Named, NFData)

This is the wrong representation, as it neglects to account for the
fact that options could be added to the diff_term stanza. A sum type
does not provide enough constructors to handle the case of when none
of deleted, inserted, replaced etc. is Just anything. A more
correct definition follows:

data DiffTreeVertexDiffTerm = DiffTreeVertexDiffTerm
  { deleted :: Maybe DeletedTerm
  , inserted :: Maybe InsertedTerm
  , replaced :: Maybe ReplacedTerm
  , merged :: Maybe MergedTerm
  }

This patch applies the above change, using -XPatternSynonyms to
provide backwards-compatible API shims. Though this changes JSON
output format (through the ToJSON instance), it should have no
bearing on backwards compatibility in protobuf objects, since there is
no way to consume diff trees as protobufs as of this writing.

Fixes #168.

[robrix]

Does the above comment imply that our pattern synonyms will be overwritten?

[patrickt]

Yes, unfortunately (though we haven't regenerated this file in ages). I'm going to file a bug with the upstream proto3-suite to add generation of pattern synonyms for oneof types.