This method merges the contents of the specified message into the
current message. Singular fields that are set in the specified message
overwrite the corresponding fields in the current message. Repeated
fields are appended. Map fields key-value pairs are overritten.
Singular/Oneof sub-messages are recursively merged. All overritten
sub-messages are deep-copied.
In php, class name cannot be "Empty". Modified code generator to (#2375) generate GPBEmpty instead (for google.protobuf.Empty only). Also change
runtime code to work with the new generated code accordingly.
This pull request includes two implementation: C extension and PHP
package. Both implementations support encode/decode of singular,
repeated and map fields.
