Log Message:
devel/go-protobuf: Update to 1.2.0

Notable changes:

- add DiscardUnknown function
- The serialization logic (for both Marshal and Unmarshal) has been
  optimized. Testing inside Google demonstrate that the new
  implementation is about 1.3x to 2.1x faster.
- Unknown fields in proto3 are now preserved.
- JSON marshal and unmarshal now check for proto2 required field.
- GetExtension returns the raw bytes if the ExtensionDesc does not
  have type information.
- Invalid UTF-8 in string types are rejected during marshal and
  unmarshal.
- Fields of bad wire types are treated as unknown fields instead of an
  error.
- The protoc-gen-go generator supports the paths=source_relative
  option.
- Unknown fields in map entries are now ignored instead of causing an
  unmarshal error.
- The "import public" feature is now implemented using Go type
  aliases.
- jsonpb.Unmarshal correctly handles escaped strings for some of the
  well-known types
- jsonpb.Unmarshal correctly returns an error when trying to marshal a
  scalar enum into repeated enums.
- jsonpb.Unmarshal correctly handles string versions of numeric
  primitives
- jsonpb.Unmarshal correctly handles enum values in maps
- fixes a performance regression where nested maps led to quadratic
  behavior
- reverts strict UTF-8 validation for proto2 string fields
- makes UTF-8 validation errors for proto3 string fields non-fatal