cmd/protoc-gen-go: additional conflict resolution for oneof field types

This is purely for consistent output with the previous protoc-gen-go.

Consider this message:

  message M {
    oneof union { string conflict = 1; }
    message Conflict {}
  }

The type for the wrapper of M.conflict will have the same name as the
embedded message type for M.Conflict.

The previous protoc-gen-go performs a disambiguation step where it adds
_s to the names of oneof field types until they have no conflicts with
nested messages or enums in the same message as the field.

There are a number of ways in which this can fail, of course. Preserve
the behavior for now.

Change-Id: I78a1c6588b577324e003b8bc337b75bb363432ba
Reviewed-on: https://go-review.googlesource.com/136357
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/cmd/protoc-gen-go/main.go b/cmd/protoc-gen-go/main.go
index 8c73c0c..2c94298 100644
--- a/cmd/protoc-gen-go/main.go
+++ b/cmd/protoc-gen-go/main.go
@@ -526,7 +526,7 @@
 		defaultValue := fieldDefaultValue(g, message, field)
 		g.P("func (m *", message.GoIdent, ") Get", field.GoName, "() ", goType, " {")
 		if field.OneofType != nil {
-			g.P("if x, ok := m.Get", field.OneofType.GoName, "().(*", message.GoIdent.GoName, "_", field.GoName, "); ok {")
+			g.P("if x, ok := m.Get", field.OneofType.GoName, "().(*", fieldOneofType(field), "); ok {")
 			g.P("return x.", field.GoName)
 			g.P("}")
 		} else {