git2go: Use gob for submodule sub-command invocation

So far we used JSON serialization to invoke submodule sub-command of the git2go executable. We are moving towards usage of the gob serialization to do the same and use stdin and stdout of the process to get an input and return the result of the execution. gob doesn't require us to do mapping of the fields and allows passing typed errors between the processes.

In order to support smooth migration we should support both serialization formats. We can drop support of the JSON in the next release.

Part of: #3232 (closed)