Make ObjectHash-Proto compatible with the latest protoc-gen-go.

This is done by ignoring the additional internal non-content fields that
were added in the latest version.

Specifically, the following fields are now ignored:

- "XXX_NoUnkeyedLiteral": This is an empty struct whose presence is used
  by the proto library to force users to use keyed literals to ensure
  forward compatible usage of messages.

- "XXX_sizecache": This is used in serialization. Its value is
  irrelevant to the actual content of the proto and should not be used
  in comparisons or for generating hashes.

detection.go

@@ -119,6 +119,21 @@ func isAProto2BytesField(v reflect.Value, sf reflect.StructField) bool {
 	return true
 }
 
+// isContentIndependentField checks if a proto field is not related to its
+// contents.
+//
+// Content independent fields include:
+// - "XXX_NoUnkeyedLiteral": This is an empty struct whose presence is used by
+//   the proto library to force users to use keyed literals to ensure forward
+//   compatible usage of messages.
+// - "XXX_sizecache": This is used in serialization. Its value is irrelevant to
+//   the actual content of the proto and should not be used in comparisons or
+//   for generating hashes.
+func isContentIndependentField(v reflect.Value, sf reflect.StructField) bool {
+	name := sf.Name
+	return name == "XXX_NoUnkeyedLiteral" || name == "XXX_sizecache"
+}
+
 // isUnset checks if the proto field has not been set.
 //
 // This also includes empty proto3 scalar values.

object_hasher.go

@@ -163,6 +163,11 @@ func (hasher *objectHasher) hashStruct(sv reflect.Value) ([]byte, error) {
 		v := sv.Field(i)
 		sf := st.Field(i)
 
+		// Ignore content-independent "XXX_" fields.
+		if isContentIndependentField(v, sf) {
+			continue
+		}
+
 		// Ignore unset fields (and empty proto3 scalar fields).
 		unset, err := isUnset(v, sf)
 		if err != nil {