merging protos created from the same source should be a no-op

[vchudnov-g]

In the course of working on #112 , I discovered that merging a synthetic proto with itself, which one would expect to be a no-op, actually changes the protos.

Two ways to see this;

1. In src/test/java/com/google/cloud/discotoproto3converter/DiscoToProto3ConverterAppTest.java, re-enable the assertEquals in convertWithIdempotentMerge and examine the test failure. Some fields get extra comments.
2. Use the reproduction steps in merging protos with Any fields generates a fictional, invalid proto field #113, which is related