Make git_config_parameters a repeated string instead of bytes

It's a bit of pain to have to deal with raw bytes when dealing with these parameters with gitlab-shell. In gitlab-shell, instead of passing in a string JSON argument like so:

./go_build/bin/gitaly-upload-pack foo '{"git_config_parameters":["key=value"]}'

We have to write raw bytes such as the following:

./go_build/bin/gitaly-upload-pack foo '{"git_config_parameters":[123 456 392]}'

The alternative would be to get the input data via an environment variable instead.