Use gob to serialize conflicts command

So far we used JSON serialization to invoke conflicts 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.

While on it the change also includes a fix to the improper error check and more detailed error generation by including proper error code depending on the root cause of the error.

Part of: #3232 (closed)